Former foreign secretary Boris Johnson has denounced a mooted Brexit deal with the EU as an "absolute stinker" and urged MPs to reject it.
The Brexit figurehead was responding to reports that Prime Minister Theresa May is close to striking a deal with Brussels which would allow the creation of a whole-UK customs union, avoiding the need for the Northern Ireland border "backstop" that has been at the heart of the impasse in negotiations.
Cabinet minister James Brokenshire rejected suggestions that final agreement has been reached, saying negotiations are "still very firmly continuing", with 95% of issues resolved.
But expectations are rising that UK negotiator Olly Robbins will be pressing hard to finalise a deal in Brussels this week, to set the scene for a special Brexit summit later in the month to secure the approval of the leaders of the 27 remaining member states.
Mrs May is expected to brief the Cabinet on progress in talks when members gather for their weekly meeting at 10 Downing Street on Tuesday.
Meanwhile, the Daily Telegraph reported that Brexit Secretary Dominic Raab has privately demanded the right to pull Britain out of the EU's proposed Irish backstop after just three months.
Reports at the weekend suggested that the EU is now ready to contemplate concessions which would keep all of the UK in a temporary customs union following the end of a transition period due to end on December 31 2020.
At the same time, the Sunday Times reported that the Prime Minister was on course to agree a future economic partnership that would leave open the possibility of a Canada-style free trade deal sought by Brexiteers.
But Mr Johnson dismissed this format as "a Christmas present of the finest old Brussels fudge" which would mean the UK getting "absolutely nothing in return" for its £39 billion divorce payment.
The UK would effectively become a non-voting member of the EU, forced to accept laws made in Brussels with no power to shape or amend them, he said.
Writing in the Sun, Mr Johnson described the arrangement as a "national humiliation", leaving the UK as "a vassal state, a colony".
"From social policy to the environment, from agriculture to industrial standards to immigration, when Brussels says 'Jump', the British answer will be 'How high?'" he said.
Mr Johnson dismissed suggestions that the arrangement would lead the way to a Canada-style trade deal, arguing that the joint UK/EU political declaration on the future relationship due to be released alongside the withdrawal agreement will be "worthless".
Brussels would always use the prospect of a hard border in Ireland to keep the UK "forever" in its orbit, he claimed.
"We are being asked to choose between the break-up of the Union – at least for economic purposes – or the subjugation of the whole country," said the Uxbridge MP.
"We are choosing wholesale subjection. We need to stop before it is too late."
Insisting that the UK should "junk" any solution keeping it in the EU's customs union, he said: "Brexit was meant to be about taking back control. Under this plan we surrender control to Brussels.
"As soon as MPs understand what is really at stake, I have no doubt that they will throw this deal out."
Speaking ahead of the publication of Mr Johnson's comments, a Downing Street spokeswoman said: "The Prime Minister is clear we are leaving the customs union.
"We are making good progress on the future relationship, and 95% of the withdrawal agreement has been settled. Negotiations are ongoing."
A spokesman for Ireland's Tanaiste Simon Coveney said: "The UK has given written commitments last December and March that the withdrawal agreement will include a legal guarantee of no return to a hard border in Ireland in any circumstance.
"In March the UK agreed this backstop will apply 'unless and until' a close future relationship eliminates any need for border infrastructure or related checks and controls.
"While we too hope the Northern Ireland backstop will never be required to be used, it will be required to be written down in legal text.
"This has been committed to by the UK in order to have a withdrawal agreement. We hope a deal can be done but we're not there yet."