Boris Johnson is heading to Brussels for a crunch EU summit on Thursday as he aims to draw a line under three years of talks and deliver a Brexit deal.
But his plans were dealt a potentially fatal blow before he'd even left as his key DUP allies said they could not support what's on the table at the moment.
The DUP's Arlene Foster and Nigel Dodds said they "could not support what is being suggested on customs and consent issues" and that there is a "lack of clarity on VAT".
They added the party will continue to work with the Government to try to get a "sensible" Brexit deal
Negotiations before the summit of the 28 EU leaders took place on Wednesday as arrangements for the Irish border continue to prove troublesome.
The DUP is unhappy with the prospect of a customs border between Northern Ireland and the rest of the UK, as well as issues of consent regarding the suspended Stormont assembly.
Mr Johnson has spoken to European Commission president Jean-Claude Juncker over the phone before travelling to Brussels.
The pair are due to meet at the European Council summit, where there is hope of a Withdrawal Agreement being signed off.
"The Prime Minister spoke with Jean-Claude Juncker this morning as part of the ongoing discussions on securing a Brexit deal," said Mr Johnson's spokesman.
Another sticking point in the proposed agreement relating to customs and VAT.
Mr Johnson will be reliant upon his colleagues within the Conservative party and elsewhere on the green benches of the Commons to get any deal through Parliament as he does not hold a majority.
He needs to get an agreement approved at the summit if he is to avert a major political bust-up over asking Brussels to delay Brexit beyond the scheduled October 31 deadline.
A compromise could see the leaders looking at a political agreement, rather than a legal text.
First Vice-President of the European Parliament Mairead McGuinness told ITV's Peston Show that teams of negotiators for both the European Union and the UK will work throughout the night in a bid to make a breakthrough before dawn.
She said if no agreement by a deadline of 8am local time (7am BST) can be made, the Vice-President said, then there will be nothing for EU leaders to discuss.
With no Brexit deal yet agreed on, an extension is looking more and more likely.
However, the PM could take some comfort from the stance of arch-Brexiteer Conservative MP Steve Baker.
After a meeting in Downing Street, the chairman of the pro-Brexit European Research Group told Sky News: “We have made great progress in our discussions with Number 10.
“We know there will be compromises, but we will be looking at this deal in minute detail, with a view to supporting it.
“But until we get that text, we can’t say.”
During a brief address to the 1922 Committee of Tory backbenchers on Wednesday, Mr Johnson compared the situation to climbing Mount Everest, according to MPs who attended.
Referring to the Prime Minister, leading Brexiteer Mark Francois said: “He said, ‘We are not quite at the summit, we are at the Hillary Step’.
“‘The summit is not far but at the moment there is still cloud around the summit.'”
Earlier on Wednesday, Brexit Secretary Stephen Barclay confirmed that Mr Johnson will write a letter asking for an Article 50 extension if no deal is in place by Saturday, something the Prime Minister has repeatedly ruled out.
The PM has promised that Britain will quit the bloc on Halloween “do or die” and any extension would be the third after the EU pushed the Brexit date back twice from March 29 and June 30.
DUP leader Arlene Foster moved to reject a suggestion that her party had accepted the latest proposals in the deal regarding Northern Ireland consent issues.
She tweeted: “Discussions continue. Needs to be a sensible deal which unionists and nationalists can support.”