Theresa May has been warned that her Brexit transition deal will be voted down in the House of Commons unless she tears up "unacceptable" proposals for fishing.
Some 14 of the PM's backbench parliamentary allies - 13 Conservatives and one DUP MP - 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 for the European Council summit in Brussels on March 22-23, clearing the way for crucial talks on post-Brexit trade to begin in earnest.
But the 14 MPs, including leading backbench Brexiteer Jacob Rees-Mogg, said the proposal for Britain effectively to remain in the EU's Common Fisheries Policy for almost two years after Brexit day in March 2019, with no say over the allocation of quotas, would not command the support of the Commons.
"These demands are completely unacceptable and would be rejected by the House of Commons," they said.
Two of the Conservative signatories, Thanet South's Craig Mackinlay and Aberdeen South's Ross Thomson, were joining fisherman to dump fish into the Thames opposite Parliament in protest against the deal.
However the stunt appeared to be running into difficulties after their boat, the Holladays R8, was unable to moor at Embankment Pier, where Mr Rees-Mogg was speaking to reporters.
At this week's summit 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.
But in the Commons on Tuesday a series of angry Tories rose to denounce the agreement hammered out between Brexit Secretary David Davis and EU negotiator Michel Barnier.
Environment Secretary Michael Gove - a leading figure in the Leave campaign - expressed his "disappointment" at the EU side's unwillingness to move on the issue.
In a sign of Government unease about the reaction, Theresa May met MPs with fishing ports in their seats in an attempt to explain their approach.
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 starting on Thursday.
The leaders of the remaining 27 had been expected to rubber stamp the agreement - including the transitional arrangements - finalised in the Belgian capital on Monday.
But in his letter formally inviting them to the meeting, Mr Tusk said: "Whether all 27 member states can welcome this at the European Council remains open. I still need a couple more hours to consult with some of the most concerned member states."
Failure to secure agreement on the terms of the UK's withdrawal would be a bitter setback for Mrs May, casting major doubt on her goal of getting broad agreement on Britain's future relationship with the EU, including a free trade deal, by October.
Downing Street insisted they had made "very good progress" in reaching agreement with the European Commission on the transition.
The Prime Minister's official spokesman said: "The European Commission has been clear, as has the European Council, that getting a deal is in the interests of not only the UK but businesses and people across the EU."