The chief Brexit adviser to British Prime Minister Theresa May has reportedly told her in a secret letter that the backstop agreed in the withdrawal deal represents a "bad outcome" for Britain.
The correspondence - leaked to the UK's Daily Telegraph - warned Mrs May that the backstop would mean the imposition of regulatory checks between Northern Ireland and the rest of the UK.
Oliver Robbins warned there is no legal "guarantee" that Britain will be able to break off from the backstop, potentially leaving the UK in a customs union with the EU.
The backstop is intended to prevent the return of a hard Irish border.
Meanwhile, Mrs May has been warned she risks creating a "constitutional crisis" in the UK, if the government fails to release the full legal advice it has received on the Brexit agreement.
Opposition parties there are threatening to launch contempt of parliament proceedings against Downing Street over the issue, with MPs voting last month to ensure the government would be required to lay before parliament "any legal advice in full", including that given by Attorney-General Geoffrey Cox.
Ministers have since indicated that only a "full reasoned political statement" will be released.
Northern Ireland's DUP - which props up the minority Conservative government at Westminster - is said to be ready to help prompt contempt proceedings, which would pile yet more pressure on Mrs May with little more than a week to go before parliament votes on the deal she struck last month.
Mr Cox will make a statement to the House of Commons and answer questions from MPs on Monday, but it is unlikely to satisfy leading figures on both sides of the Brexit debate.
December 11th vote
Mrs May - who will address the Commons on Monday afternoon to update MPs on the G20 summit in Argentina - still faces an uphill task to save her Brexit deal.
Labour's Brexit secretary Keir Starmer has said it was "inevitable" the party would try to oust Mrs May if she loses a potentially historic vote on December 11th - two days after she is due to take part in a TV debate with Jeremy Corbyn.
There is still disagreement over the format of the programme, with the UK Labour leader indicating that he will only go with the Conservatives' preference for a BBC debate if it is a straight head-to-head.
BBC also wants leaders to take questions from a wider panel and Downing Street has accused Mr Corbyn of "running scared" by so far refusing to sign up.
There will also be no room on the debate podium for SNP leader Nicola Sturgeon, who is travelling south to London in a bid to persuade MPs to try and secure an extension to Article 50.
"Best deal is remaining in the EU"
Speaking on Sunday, Ms Sturgeon said: "The last week has been a watershed moment in the Brexit process.
"Having now finally published its own official analysis of the economic consequences of Brexit, the UK government can no longer hide behind their own spin - it is clear that any kind of Brexit will make Scotland and the UK poorer.
"The reality is the best deal is remaining in the EU, which is exactly what the people of Scotland voted for."
Despite it all, UK Environment Secretary Michael Gove said on Sunday he believed the government could still win the parliamentary vote next week.
Mr Gove, one of the leaders of the Leave campaign in the 2016 referendum, urged fellow Brexiteers to back the agreement warning the alternative was "no deal or no Brexit".
He said that if the government lost the vote there was a "real risk" there could be a majority in the Commons for a second referendum.
