How the battle over fishing rights could scupper Theresa May's Brexit deal

• Fishing rights have become a major issue in Brexit negotiations.
• The UK government this week pleaded with Conservative MPs who believe May's Brexit deal sacrifices UK fishing industry, to back her deal.
• The issue could be a major stumbling block to the prime minister securing a future trade deal.

LONDON - Britain's fishing industry may be relatively small, accounting for less than half a percent of UK GDP, but it has played an outsized role in Brexit negotiations.

This is partly because it is such a symbolic industry for an island nation like Britain and partly because it stands to be re-shaped more radically by Brexit than perhaps any other sector. It could also yet be an issue which helps to derail Theresa May's chances of securing a trade deal with the EU in the next few years.

Here's why

The issue is simple enough. After Brexit, the UK will reclaim sovereign control of its waters, which for decades have been subject to the EU's deeply unpopular Common Fisheries Policy (CFP).

British fishermen say the CFP represents a raw deal for their industry. Ever since the policy was implemented, the UK has received a disproportionately small share of Europe's fishing stocks. Its boats get 9% of English Channel cod, even though almost all the catches are made in UK waters. French boats, by comparison, get 84%.and its members overwhelmingly supported Brexit as a means of putting an end to Britain's participation in the policy.

Many in the fishing industry hoped that Brexit would see the UK restrict access to foreign vessels and manage its fish stocks independently. But the EU is fiercely opposed to that plan and intends to demand access to British waters for European vessels in return for a free trade deal.

That dispute will need to be resolved before either side can agree on a trade deal. For her part, Theresa May told MPs on Thursday that she had "firmly rejected" EU demands for access to British waters in return for a trade deal, and said the political declaration - a document which outlines the UK and EU's aspirations for their post-Brexit relationship - was a win for fishermen.

But moments later, the EU's deputy chief Brexit negotiator Sabine Weyand shot back, tweeting that a fisheries agreement was "in the best interest of both sides," sharing research which suggested Britain needs imports of EU cod for its fish and chips, not mackerel and herring.

Only one side can win that dispute. Given the precedent of the last two years, the EU will be confident that the UK will eventually cave in to its demands and agree to some form of cooperative agreement. And it could well be in Britain's interests, too: If the UK closed its waters to EU vessels, the EU could simply respond by slapping tariffs on UK exports of fish. As the UK exports 80% of its fish to European waters, that would be a huge problem.

The pressure on Theresa May over the row is set to escalate this weekend. A leaked document due to be agreed at a summit of EU leaders on Sunday will declare that the post-Brexit arrangements over fishing rights will build on the much-hated current arrangements, saying any future agreement must protect the rights of European fishing fleets.

Much is at stake: The document goes onto warn that failure by Britain to come to an agreement on the issue could result in the EU refusing to grant the UK an extension of the transition period, which Theresa May is likely to require in order to negotiate an EU trade deal and avoid the unpopular backstop.

The government, for its part, insists the UK has no intention of maintaining existing access. But if it is the price of a trade deal, further concessions may well be on the way.