Theresa May will hold crunch talks with Emmanuel Macron today as the Brexit deal hangs in the balance amid DUP fury at her 'betrayal'.
The Prime Minister is meeting the French President for lunch as the drama ramps up - with signs a divorce package is on the brink of being agreed.
But domestic pressure on Mrs May is mounting after the DUP - which is propping Mrs May up in power - claimed she is preparing to break her promise never to sign off a deal which risks splitting Northern Ireland from the rest of the UK.
The row erupted after a leaked letter from the premier to the party's leader Arlene Foster suggested she is ready to make concessions on EU demands over the Irish border 'backstop'.
The principle of the backstop was included in the draft divorce deal struck last December, and is intended to avoid a hard border between Northern Ireland and the Republic.
It would only come into effect if no wider trade agreement is sealed by the end of a mooted transition period in December 2020.
But the EU and UK have proposed significantly different versions - and thrashing out a compromise is the last stumbling block in the withdrawal talks.
The letter said Brussels is still pushing for its backstop - which would see Northern Ireland stay effectively within the bloc's customs and regulatory jurisdiction - to be included in the Withdrawal Agreement as an insurance policy in case the UK's all-UK backstop does not work.
Mrs May insisted in her missive, written on Tuesday and leaked to The Times, that she will never allow a divide in the Irish Sea to 'come into force'.
Irish border backstop mechanism is the final hurdle in divorce talks
The Brexit divorce negotiations have boiled down to the issue of the Irish border.
Brussels had initially demanded that Northern Ireland stays within its jurisdiction for customs and most single market rules to avoid a hard border.
But Mrs May flatly rejected the idea, saying she would not agree to anything that risked splitting the UK. Instead, the government has mooted a temporary customs union for the whole UK, and accepted the need for extra regulatory checks in the Irish Sea.
Brussels has also given ground, and now appears to be prepared to sign off a UK-wide backstop in the divorce deal.
That leaves the mechanism for ending the backstop as the final hurdle to overcome - but the two sides have different views.
Dominic Raab has been arguing that the UK should be able to scrap the backstop arrangements by giving three to six months' notice.
That would assuage Eurosceptic fears that the country could end up being trapped in an inferior customs union indefinitely, unless the EU gives permission for it to stop or a wider trade deal is sealed.
For its part, the EU has been adamant that the backstop must offer an 'all-weather' solution to the Irish border issue and stay in place 'unless and until' it is superseded by other arrangements.
The bloc has already effectively killed off calls for a hard end date to the backstop - and No10 is now convinced that a simple unilateral notice period will not unlock the talks.
Mrs May and Irish PM Leo Varadkar have discussed a 'review mechanism' for the backstop, which could involve an independent arbitration body assessing whether the terms were being honoured and if the arrangement should be ended.
Potentially this could provide a solution that allows Mrs May to say the backstop would not go on for ever.
But the devil will be in the detail, and ministers are keen to ensure there are 'robust' ways for the UK to escape.
'I am clear that I could not accept there being any circumstances or conditions in which that 'backstop to the backstop', which would break up the UK customs territory, could come into force,' she wrote.
'That is why it is critical that the provision for a UK-EU joint customs territory is legally binding in the Withdrawal Agreement itself, so that no 'backstop to a backstop' is required.'
However, the DUP has interpreted the wording as meaning that the EU's version of the backstop could go into the agreement.
Ms Foster said: 'The Prime Minister's letter raises alarm bells for those who value the integrity of our precious Union and for those who want a proper Brexit for the whole of the UK.
'It appears the Prime Minister is wedded to the idea of a border down the Irish Sea with Northern Ireland in the EU single market regulatory regime.'
The DUP's Brexit spokesman Sammy Wilson said the PM appeared to be considering 'total betrayal' of her promises.
Tory Eurosceptics said the spat showed Mrs May's approach 'will not work'.
Downing Street tried to cool the tensions, with a spokesman saying: 'The Prime Minister's letter sets out her commitment, which she has been absolutely clear about on any number of occasions, to never accepting any circumstances in which the UK is divided into two customs territories.
'The Government will not agree anything that brings about a hard border on the island of Ireland.'
Culture Secretary Jeremy Wright said the government 'understood the concerns' of the DUP and would not accept a deal with the 'component' of splitting Northern Ireland and mainland Britain.
He told BBC Radio 4's Today programme: 'Were there to be a border down the Irish Sea, as I've said and the Prime Minister has said, we wouldn't accept a deal incorporating that.
'What's important is we get a deal that satisfies our requirements - and by our requirements I mean the whole of the United Kingdom, including Northern Ireland. 'We don't have that proposed deal yet. When we do we'll be able to discuss it.
'There is no doubt we have understood the concern the DUP and others have expressed about a hard border down the Irish Sea, we share it, and we will not accept a deal that involves that component.'
Mrs May needs the ten DUP votes to help get her Brexit plan through the Commons.
Tory Eurosceptic Jacob Rees-Mogg told The Times: 'None of this works at all.
'To say, 'because I've been incompetent in the negotiation so far I'm going to have to agree to a final backstop that could undermine the integrity of the UK, but go on to say, trust me because I'll be more competent in my future negotiation than I have in the ones to date', is a line that lacks credibility.'
Sam Lowe, from the Centre for European Reform, said the acceptance of a backstop is good news for withdrawal negotiations but agreed it will cause tensions between the prime minister and the DUP.
It came as Mrs May embarked on a diplomatic tour of Europe as she raced to get a Brexit deal ready to present to her Cabinet as soon as Monday.
She will hold meetings today with Belgian prime minister Charles Michel before lunch with Mr Macron and attending Armistice commemorations.
EU chiefs yesterday suggested a Brexit deal could be finalised by the start of next week – paving the way for it to be rubber-stamped by leaders at a special summit on Sunday, November 25.
In February, Mrs May told the Commons that the draft backstop proposed by the EU 'would, if implemented, undermine the UK common market and threaten the constitutional integrity of the UK by creating a customs and regulatory border down the Irish Sea, and no UK prime minister could ever agree to it'.