In joining the EC in 1973, the UK became subject to the body of European laws and treaties. The significance of this grew over time in two ways. First, the process of European integration continued, if sometimes sporadically. This meant that European bodies increased in importance. The European Community, for example, was transformed into the European Union (EU) in 1993. Second, the implications of EC/EU membership gradually became more apparent, as the higher status of European law over UK statute law was gradually recognised. This has led to a debate (examined below) about the continued significance of parliamentary sovereignty in a context of EU membership. The most important EU laws and treaties include the following:

• Treaty of Rome 1957. This was the founding treaty of the then European Economic Community, and all member states, whenever they joined, become subject to it.

• Single European Act 1986. This was the treaty that established a single market within the EC, ensuring the free movement of goods, services and capital.

• Treaty on European Union (TEU or the MaastrichtTreaty) 1993.This treaty introduced political union in the form of the EU, although the UK negotiated an opt-out on the issue of monetary union (membership of the euro) and, initially, on the Social Charter.

Treaty of Lisbon 2009. This was a modified version of the proposed Constitutional Treaty, which would have created an EU Constitution, incorporating and replacing all other treaties. Lisbon, nevertheless, introduced new decision-making arrangements within the Union.