Theresa May's Brexit deal in crisis as Scottish Conservative MPs threaten to rebel over fishing rules

Scottish Tory MPs have plunged Theresa May into a fresh crisis over her Brexit deal, by warning they will rebel if it leaves the UK tied to EU fishing rules.

All 13 Conservatives north of the border – including David Mundell, the Scottish secretary – signed a hand-delivered letter to the prime minister, as she began her crunch cabinet meeting.

It was sent after reports that the draft agreement will park the controversy over fishing rights until the negotiations on a future trade deal, which won’t start until next year.

In the letter, Mr Mundell and his 12 fellow Tories said Ms May had promised Britain would have “complete control and full sovereignty over domestic waters”, from the end of 2020, when the transition period ends.

It reads: “We could not support an agreement with the EU that would prevent the UK from independently negotiating access and quota shares.

“That would mean that we would not be leaving the CFP [commons fisheries policy] in practice and would be becoming an independent coastal state in name only.”

The letter sparked fresh speculation that Mr Mundell is poised to resign unless the prime minister backs down and seeks better terms on fishing rights in the draft deal.

The Scottish secretary – Ruth Davidson, the Tory leader north of the border – had already threatened to walk out if Northern Ireland is given preferential trading terms with the EU.

Even worse for Ms May, losing the support of the 13 Scottish Tories would make it even less likely that parliament will back the agreement, in a landmark vote expected early next month.

Pro-Brexit Tories have already joined forces with the Democratic Unionist Party (DUP) to warn the deal is doomed, with Labour also supplying the numbers to vote it down.

EU member states with big fishing industries have demanded access to British waters, in return for the UK enjoying tariff-free access to EU markets under plans for the Irish border ‘backstop’.

As a result, the controversy has been deferred until the end of the transition period, until a future UK-EU agreement is reached on reciprocal access to waters.

In the Commons, a Tory backbencher, Martin Vickers, demanded “an absolute assurance that it will be for the UK, and the UK alone, to determine who fishes in our national waters after a deal is signed”.

In response, the prime minister said: “I can reassure my honourable friend that we will become an independent coastal state, and it will be the UK negotiating on the UK’s behalf in terms of access to UK waters.”

However, Ms May did not say when the UK will regain negotiating rights – which, the Scottish Tories fear, will not be until after the start of 2021.

The Tories are under pressure because the SNP is opposing Northern Ireland effectively staying in the EU single market – to avoid a hard Irish border – while Scotland is forced to leave.

Ms Davidson and Mr Mundell fear it will bolster the case for independence north of the border, if the backstop creates distinct single market terms for Northern Ireland only.