at the centre of a landmark ruling in the 1990 Factortame case. A Spanish fishing company, Factortame, sued the UK government for restricting its access to UK waters. The Law Lords, who formed the highest UK court at the time, followed the European Court of Justice in ruling that the 1988 Merchant Shipping Act, which the government was using to support its position,could not be allowed to stand because it violated EU law. This controversial case established the primacy of EU law over an act of Parliament.
Fishing was one of the final sticking points in the post-Brexit trade talks. While fishing is a tiny part of the economy on both sides of the Channel, it carries big political weight.
Regaining control over UK waters was a key part of the Leave campaign in 2016.
What's the deal in a nutshell?
EU boats will continue to fish in UK waters for some years to come
But UK fishing boats will get a greater share of the fish from UK waters
That shift in the share will be phased in between 2021 and 2026, with most of the quota transferred in 2021
After that, there'll be annual negotiations to decide how the catch is shared out between the UK and EU
The UK would have the right to completely exclude EU boats after 2026
But the EU could respond with taxes on exports of British fish to the EU or by denying UK boats access to EU waters
What's the detail on fishing?
The deal runs to more than 1,200 pages, with a whole section and several annexes dedicated to fisheries.
Both sides have agreed that 25% of EU boats' fishing rights in UK waters will be transferred to the UK fishing fleet over a period of five years.
This is known as the "adjustment period", giving EU fleets time to get used to the new arrangements. The EU wanted it to be longer, the UK wanted it to be shorter - it looks like they've met somewhere in the middle, with an end date of 30 June 2026.
Under plans outlined in the deal, EU fishing quota in UK waters will be reduced by 15% in the first year and 2.5 percentage points each year after.
By 2026, it's estimated that UK boats will have access to an extra £145m of fishing quota every year. In 2019, British vessels caught 502,000 tonnes of fish, worth around £850m, inside UK waters.
The document also sets out details of how each species of fish will be shared out between the UK and the EU during the transition.
The UK fleet can expect increases in quota for 57 out of the 90 types of fish caught in UK waters every year.
But quota shares for some species like Channel cod, of which EU boats (mainly from France) catch more than 90% each year, will remain unchanged.
After the end of the adjustment period in June 2026, there will be annual talks to set the amount EU fishing boats can catch in UK waters (and vice versa).
At that point, the UK has the right to completely withdraw EU boats' access to UK waters. But the EU could then suspend access to its waters for UK boats or impose tariffs (taxes) on fish exports from the UK to the EU.
Tariffs could even be extended to other goods, but it would have to be in proportion to the economic impact of the fishing breach.
There will be an arbitration system to try to resolve fishing disputes.