The impact of the EU & Brexit on the fundamental principles of the UK constitution and Parliamentary Sovereignty 

How did membership of the EU change the UK?

EU Treaties became part of the UK's constitution

The process of making treaties is how the EU changes its structure and rules. There is no overall constitution so the treaties add up to the constitution of the EU. The treaties have allowed the EU to move to increasing integration with the development of the EU taking it from a primarily economic organisation to an increasingly political organisation. This process meant a steady transfer of elements of sovereignty from the UK  to the EU.

When the UK joined the EU it became subject to EU law. The Factor Tame Case established that EU law is supreme, however, EU laws are limited to specific areas and limited by the need to find agreement across the EU by unanimity or by qualified majority. The fact that the EU law applies in the UK has had a significant impact on policy areas such as environmental policy, health, and equality. It has also led to an unceasing sense that the UK has lost sovereignty.

EU membership enhanced devolution.

 Several policy areas in which the EU shares competence with the UK were devolved to Scotland, Wales, and Northern Ireland, including agriculture, fisheries, and the environment. On the subject of the UK's overall relationship with the EU, the central government had to consult the administrations in Edinburgh, Cardiff, and Belfast in order to produce an agreed negotiating position. This is why some writers described the UK as having a system of 'multi-level governance', in which policy-making is shared between the national government, the sub-national bodies in Scotland, Wales, and Northern Ireland, and the institutions of the EU. In this sense membership of the EU has cemented devolution into the working processes of the UK.

Individual rights and freedoms

The EU’s promotion of individual rights and freedoms across all member states has had mixed success. On the one hand, the principle of the freedom of movement has allowed citizens to move freely across borders in search of better and higher-paid jobs. Yet this in turn stoked tensions in countries such as the UK, which has seen high levels of immigration, especially from Eastern Europe. Fears over growing immigration numbers played an important part in the increasing support for Brexit.

Case study The struggle over Brexit

The timeline story of Brexit

• How has former membership of the EU affected the UK constitution? 

Brexit has led to significant constitutional issues and revealed unresolved tensions within UK democracy. Faced with strong anti-Brexit sentiment within the Conservative Party, Boris Johnson expelled 21 pro-European Conservatives. Only four of them were reelected in the 2019 general election. As a result, the Conservative Party has shifted to become a fully Eurosceptic party, leaving pro-European Conservatives without a political home. 

It can be argued that Brexit has not strengthened parliament but has instead allowed the executive branch to increase its power. For instance, when Parliament refused to agree to an early general election in 2019, the Johnson government passed a law giving the prime minister the authority to set the election date. 

The Supreme Court clashed with the executive in two Gina Miller cases. In the first case in 2017, the court ruled that Parliament, not the executive, must decide to withdraw the UK from the EU. In the 2019 case, the court stated that the Johnson government could not suspend Parliament as it impeded its right to debate Brexit. These cases led to accusations that the Supreme Court was causing delays in Brexit and becoming too involved in politics.

• How have Scotland and Northern Ireland responded to Brexit? 

Brexit damaged the United Kingdom's integrity. While 52% of voters supported leaving the EU in the 2016 referendum, the results varied among different parts of the UK. England and Wales had slight majorities for leaving, while Scotland and Northern Ireland voted decisively to remain. Scotland's pro-Europe stance has prompted the SNP to push for another independence vote with hopes of rejoining the EU. Following Brexit, the Northern Ireland Protocol was created to facilitate the movement of goods across the border with the Republic of Ireland, which remains in the EU. This has raised concerns among unionists in Northern Ireland about their UK membership. Consequently, the DUP exited power-sharing, causing the collapse of the Northern Ireland government in 2022. The disruption caused by Brexit to the UK constitution has led Labour to advocate for a significant shift in power to maintain the union. This would involve devolving power from Westminster and replacing the House of Lords with an elected Assembly of Regions and Nations.

Developments in devolution since Brexit

• How did membership of the EU influence Parliament? 

The feeling that the EU made laws without democratic oversight in the UK, and based on the limited resources available to Parliament for scrutiny of EU regulations. Parliament had a responsibility to examine EU legislation, and ministers should not have agreed to new laws unless they had been debated or reviewed by the House of Commons European Scrutiny Committee. In practice, however, the sheer volume of EU legislation makes this a difficult task to perform effectively. The House of Lords EU Select Committee produced thoughtful reports on developments in Brussels, but its status in the second chamber meant that it lacked real influence. However, while critics of the EU saw this as more evidence of the insidious influence of the EU, the majority of EU regulations were not controversial ( to anyone but the Daily Mail) and are limited to uncontentious matters to do with the environment or commerce. For example, limits on the power of vacuum cleaners or the safe disposal of fridges.

• How were Ministers and, in particular, Prime Ministers affected by the EU? 

EU membership  heightened the profile of the prime minister, 

whose annual routine has included attendance at regular European Council meetings. David Cameron visited his fellow heads of government individually early in 2016, as well as attending the Council, in a bid to renegotiate the terms of UK membership. Following the vote to leave the EU, Theresa May was thrust unexpectedly into the limelight as Britain's new prime minister, conducting a series of meetings with her counterparts in pursuit of a deal that protects vital national interests. British PMs are judged on their ability to stand up for the UK in Europe. Mrs Thatcher's status was enhanced by her success at achieving a rebate (return of money paid to the EU) and David Cameron lost status when he was seen to have failed to bring home significant concessions in the run-up to the referendum. (Cameron's EU deal)

Bexit failure undermined Theresa May.

 When Theresa May assumed office as prime minister in July 2016 after the unexpected EU referendum outcome, she garnered positive reactions from MPs and the public. Viewed as a reliable figure with vast experience from her time in the Home Office, May was widely seen as competent and accountable. Many believed she could navigate Brexit and advance the Conservative Party's objectives. However, her tenure came to an end in July 2019, influenced by Brexit and amid lingering divisions within her party and the country. What influence did Brexit have on her premiership and to what extent did it constrain her authority and accomplishments?

Boris Johnson used 'Get Brexit Done' to enhance his power. Johnson started with a minority government, facing challenges during Brexit negotiations. His failed prorogation attempt and clash with the Supreme Court in September 2019 preceded his successful call for a December 2019 election, supported by Parliament through a two-thirds majority. The UK exited the EU on January 31, 2020, marking the high water mark of Johnson's tenure as PM. The balance of power between parliament and the government changed when Johnson won an 80-seat majority at the 2019 general election. The executive is also strengthened by the extensive use of secondary legislation on Brexit policy because parliamentary scrutiny is limited here, as it is on international treaties.

• How has withdrawal from the EU affected the location of sovereignty in the UK 

As a member of the EEC/EU, the UK shared its sovereignty with other member states. The UK had to acknowledge the supremacy of European law over domestic law. The Factortame case (1991) established this principle in British law, requiring British courts to prioritize European law in cases of conflict. The question of whether parliamentary sovereignty had been compromised was resolved when Parliament initiated the process of leaving the EU in 2016, finalizing the withdrawal on 31 January 2020. This demonstrates that while the UK shared its sovereignty as an EEC/EU member, Parliament maintained the right to enact laws to fully restore parliamentary sovereignty, based on the principle that no parliament can limit its successor.

However, the outcome of the EU referendum led to conflicting assertions of sovereignty. Advocates of popular sovereignty cautioned against parliament obstructing Brexit, while proponents of parliamentary sovereignty contended that the referendum was not legally obligatory and that Brexit necessitated parliamentary approval. Brexit reinstates parliament's legislative power and diminishes the precedence of EU law, yet there are still practical constraints on the UK's capacity to assert political sovereignty while the use of referendums undermined the sovereignty of parliament. Parliament accepted the result of the EU referendum in 2016, although most MPs disagreed with it. This suggests that the political sovereignty of the public is superior to that of the Westminster Parliament.

The UK's departure from the European Union has restored full parliamentary sovereignty. The UK is no longer bound by the four freedoms of the EU, which include the free movement of goods, services, capital, and persons. The EU no longer has external sovereignty over the UK. This enables the UK to negotiate its trade agreements. However, existing EU laws have been incorporated into UK legislation through the EU Withdrawal Act.

It is also possible to argue that because of the cooperative relationships across the English Channel, the UK’s membership in the European Union has on balance enhanced rather than eroded effective U.K. autonomy. British skeptics can legitimately argue that the EU Commission and its governments may have adopted excessive regulations.  But skeptics cannot argue that those decisions were taken without U.K. participation.  Whatever future decisions may be made about Brexit, moreover, many of the existing cooperative relationships with the EU are likely to be maintained—precisely to forestall an unwanted deterioration in effective U.K. autonomy. 

However, Brexit has resulted in potential conflicts with EU member states over disputed territorial issues. For example, in 2021, the UK dispatched two warships to Jersey in response to French fishing boats threatening a blockade due to disagreements over post-Brexit fishing rights. The sovereignty of Gibraltar has also become a contentious issue, especially as the UK and Spain no longer share EU membership.

The EU–UK Trade and Cooperation Agreement was agreed on Christmas Eve 2020. Its key features were:

● There are no tariffs or quotas on goods traded between the UK and EU. But this is subject to rules of origin, meaning that goods (e.g. cars or food products) which include a high content of material produced outside the UK may face tariffs if exported to the EU.

● Non-tariff barriers, such as customs declarations and sanitary standards checks, are created.

● Tariffs can be imposed if the UK or EU believe that level playing field

provisions (e.g. on workers’ rights, environmental standards or state aid) have been breached.

● The UK’s share of fishing quotas increases by 25% for an initial five- year period. Annual negotiations on quotas will then commence.

● There is cooperation on law and security (e.g. data sharing and extraditions), but the measures are less robust than before Brexit.

● The UK participates in, and contributes to, some EU programmes (e.g. Horizon research funding) but will no longer participate in others (e.g. the Erasmus+ student exchange scheme).

● New institutional frameworks, including a partnership council, are created.

Brexit's Impact on Voting Behaviour

The 2017 and 2019 general elections were heavily influenced by Brexit. Similar to the 2016 referendum, voting behavior was influenced by the cosmopolitan versus non-cosmopolitan divide. Older, white, working-class individuals with limited educational qualifications and socially conservative views tended to favor the Conservative party, while younger, BAME, middle-class, and educated individuals with socially liberal views leaned towards Labour. Brexit allegiances often overshadowed party loyalties, with most voters aligning themselves with a party that shared their Brexit stance. Leave voters predominantly supported the Conservative party, while Remain supporters were more likely to back Labour.

Brexit and Political Parties

Divisions deepened within the two major parties following the EU referendum. May's Withdrawal Agreement faced defeat three times due to rebellions from Conservative MPs on both sides of the Brexit debate. Labour Remainers pushed for the party leadership to back a second referendum, while some Labour MPs representing Leave-voting constituencies advocated for a softer Brexit. After the 2019 general election, the Conservatives solidified their stance as a pro-Brexit party. Changes occurred within the party landscape as certain Labour and Conservative MPs switched sides to form Change UK in 2019, although the party disbanded in less than a year. The Brexit Party had a brief existence and later rebranded as Reform UK in January 2021. Support for UKIP dwindled after achieving its primary goal.

The impact of the Brexit process on the United Kingdom's Constitution

The UK's withdrawal from the European Union was a complex and protracted journey. The victory for the leave campaign in the referendum held on June 24, 2016, was followed by a lengthy process, culminating in the UK's formal departure on January 30, 2020, and the transition period ending on December 31, 2020.

The implementation of the referendum decision took approximately four and a half years, reflecting the significant political and legal challenges involved. Throughout this period, the House of Commons demonstrated its vital role in the constitutional process. Westminster Hall, a historic venue, once again became a focal point for debate, illustrating the UK's unwritten constitution, customs, and traditions through passionate rhetoric and procedural developments.

The referendum's role in shaping parliamentary action highlights its influence on the constitutional landscape. While the referendum itself is not legally binding, it has been treated as politically binding, especially on issues of national importance such as EU membership. The 1975 and 2016 referendums exemplify how internal party divisions and political considerations have shifted decision-making authority from Parliament to the electorate.

Parliament's sovereignty—the principle that it can make or unmake any law—remains central to the UK constitutional framework. This concept, articulated by constitutional scholar Albert Dicey, emphasizes Parliament's ultimate authority, which has developed over centuries in the absence of a codified constitution. However, political realities, such as divisions within Parliament and external constraints, challenge the notion of absolute sovereignty.

The Brexit process has demonstrated that political circumstances can significantly limit Parliament's legislative capacity. Although Parliament possesses the legal authority to pass any law, practical limitations arise from political divisions, the strength of the government, and public opinion. The use of referendums introduces a layer of direct democracy, which, while influential, is not legally binding but politically respected.

The composition of Parliament, comprising the elected House of Commons and the unelected House of Lords, plays a crucial role in lawmaking. During Brexit, the Lords, which largely favored remaining in the EU, exercised caution to avoid obstructing the will of the elected Commons. Their role includes challenging legislation, proposing amendments, and delaying bills, but they cannot indefinitely block legislation, especially on financial matters.

The political context, including Prime Minister Theresa May's attempt to call a snap election in 2017, affected parliamentary dynamics. The resulting slim majority and internal divisions hampered legislative progress, illustrating that parliamentary power is contingent upon political unity and strength. The role of the Speaker of the House of Commons, exemplified by John Bercow, became prominent during this period, highlighting how procedural interpretation can influence parliamentary authority.

The relationship between Parliament and the executive is characterized by the absence of a formal separation of powers. The Prime Minister and the government control the legislative agenda, originating most legislation. Historically, Parliament evolved from advisory councils to a dominant legislative body, with the Crown's role becoming ceremonial. During Brexit, this relationship was tested, Parliament defeated May's deal on three occasions as well as delaying Johnson's request for an election. This was a level of Parliamentary Independence and assertiveness not seen since the days of John Major. Far from being a rubber stamp, the Parliament began to resemble something like a US Congress however, it was never able to take control of Brexit policy, and although it blocked May's deal, it never unified around a policy of its own. Parliament was against Brexit, against soft Brexit, against a no Brexit and another referendum, although groups within Parliament supported all of these alternatives. 

Has anything changed? After Johnson's victory in the 2019 election it might seem that the regular order has been restored since he has  80 seat majority, but it may also be that Parliament has set a precedent for future assertiveness - the revolt over Huawei and the appointed of Julian Lewis to the chair of the Intelligence and Security Committee against the government's wishes might be a sign of this continued assertiveness.

2025, the Labour government, only one year in office and with a large majority, was forced to U-turn on benefit changes, in another sign that the struggle over Brexit may have had a lasting impact on backbench assertiveness.But overall, the executive maintained significant control over lawmaking.

Parliament's functions extend beyond legislation to scrutinize and challenge government actions. Committees and parliamentary questions serve as vital mechanisms for oversight. The European Union (Withdrawal) Bill exemplifies Parliament's role in scrutinizing legislation, particularly in removing EU law—over 20,000 pieces—while ensuring responsible exercise of powers by the government.

When the government commands a strong majority, it can dominate legislative processes, reducing Parliament's influence. This dynamic underscores that parliamentary sovereignty is heavily influenced by political conditions, not solely by legal authority.

The relationship between Parliament and the judiciary, especially the UK Supreme Court, has gained prominence. Unlike the US or Germany, UK courts cannot strike down legislation on constitutional grounds. Nonetheless, the Supreme Court has asserted its interpretative authority, notably during Brexit, through cases brought by Gina Miller. These cases clarified Parliament's constitutional position and the limits of executive power, exemplified by the ruling that the prorogation of Parliament in 2019 was reviewable and could be challenged in court.

This judicial assertiveness raises questions about the evolving role of the judiciary and its impact on parliamentary supremacy. While courts do not have the power to invalidate laws on constitutional grounds, their interpretations influence the balance of power within the UK constitutional system.

In conclusion, Parliament remains the primary legislative authority, but Brexit has revealed that its formal power is conditioned by political realities. Divisions within Parliament, the strength of the government, and judicial interventions all shape its effective authority. The relationship among Parliament, the government, and the courts is fundamental to understanding the UK Constitution.

External factors, such as EU membership and internal nationalist movements in Scotland and Northern Ireland, further complicate the constitutional landscape. The future of UK sovereignty depends on managing these internal and external pressures, with Parliament at the core of this ongoing process. Despite these challenges, Parliament remains one of the most influential legislatures globally, though its authority must be understood within the broader political and constitutional context, especially in light of recent developments.