A group of 14 backbench MPs, including three Scottish Conservatives and one from the DUP, have signed a joint letter denouncing the draft deal agreed by the Government earlier this week.

Mrs May is hoping the deal will be signed off at a meeting of EU leaders that begins in Brussels today, clearing the way for crucial talks on post-Brexit trade to begin.

But the 14 MPs, including Jacob Rees-Mogg, rejected the proposal for Britain to remain bound by the EU’s Common Fisheries Policy for almost two years after Brexit day in March 2019, with no say over the allocation of quotas.

Britain's Prime Minister Theresa May speaking during the weekly Prime Minister's Questions in the House of Commons. Picture: Getty Images

“These demands are completely unacceptable and would be rejected by the House of Commons,” they said.

The MPs said Mrs May should indicate her intention to take back control of the 200 nautical mile exclusive economic zone and state that “UK national fisheries resources are not negotiable”.

And they said she should tell fellow EU leaders: “Leaving the European Union means setting our own fisheries policy from March 29 2019. The UK will not remain party to the CFP during the proposed implementation period.”

Ministers insist they have agreed “specific safeguards” with Brussels over the annual negotiations on fishing quotas in 2019.

At Prime Minister’s Questions, Mrs May was asked by SNP MP Drew Hendry “what changed between last week and this week”, after Scottish Conservative leader Ruth Davidson and Environment Secretary Michael Gove wrote an article claiming “Britain will leave the CFP as of March 2019” days before the transition terms emerged.

And the Prime Minister was also challenged by her own MPs, with Anne-Marie Trevelyan calling on Mr Gove’s department to “prepare a financial mitigation plan to protect our fleet until 2021” in case EU countries “try to take advantage of our losing our voice in the CFP” during a transition.

Meanwhile, European Council president Donald Tusk has warned there is no guarantee EU leaders will accept the agreement covering the terms of Britain’s withdrawal when they meet in Brussels this week.

Mr Tusk said on Tuesday that he still needs more time to consult with “some of the most concerned member states” ahead of their two-day summit.

May told to fix fishing plans or rebels will sink Brexit deal