Boris Johnson has suffered a blow to his proposed Brexit deal as the Democratic Unionist Party said it cannot support plans “as things stand”.
The support of the DUP is seen as crucial if the prime minister is to win Parliament’s approval for the deal in time for his 31 October deadline.
The DUP said it would continue to work with the government to try to get a “sensible” deal.
It comes as Mr Johnson heads to a crunch summit to get the EU’s approval.
On the EU’s side, the legal text of a draft Brexit deal is seen as being “pretty much ready”, the BBC’s Europe editor Katya Adler said.
But the UK government has yet to approve the documents and the DUP remains unhappy about elements of the prime minister’s revised plan for Northern Ireland.
In a joint statement released on Thursday, the DUP’s leader and deputy said discussions with the government were “ongoing” but “as things stand, we could not support what is being suggested on customs and consent issues and there is a lack of clarity on VAT”.
“We will continue to work with the government to try and get a sensible deal that works for Northern Ireland and protects the economic and constitutional integrity of the United Kingdom,” Arlene Foster and Nigel Dodds added.
The party has helped prop up the Conservative government since the 2017 general election.
In the past, a number of Tory Brexiteers have said their own support for a Brexit deal was contingent on the DUP’s backing of any agreement.
The BBC’s Adam Fleming said the DUP’s rejection of Mr Johnson’s proposals has put a “big spanner in the works”.
Our Brussels reporter said the “choreography” of Thursday’s summit in Brussels had now been put in “jeopardy”.
BBC political editor Laura Kuenssberg said the DUP’s lack of support was a “but of giant proportions”.
The decision by the DUP to reject Mr Johnson’s deal is a potential body blow.
Not only would it seem to scupper the PM’s prospects of securing a deal in Brussels today, but it also suggests he would be unable to win any subsequent vote in the Commons.
In their statement, however, the DUP do not quite close the door to a possible compromise in the future, implying were there further concessions, they might be won round.
The DUP highlight the new customs regime and the issue of consent as the key sticking points.
However, its understood issues around securing the consent of Stormont to the new customs regime have become paramount.
The fear in the DUP is that under the simple majority vote required by the EU to ensure continued membership of the new customs arrangement, the unionist community would have no veto.
So they worry they could remain trapped in an arrangement where increasingly the political and economic pull would be towards Dublin and Brussels, rather than London – threatening their core belief in the union.
Mr Johnson’s proposals for a new Brexit deal hinge on getting rid of the controversial backstop – the solution to border issues agreed by former PM Theresa May which proved unpalatable to many MPs.
However, his plans would see Northern Ireland treated differently from the rest of the UK – something the DUP, among others, has great concerns about.
The DUP has also demanded assurances around the so-called consent mechanism – the idea the prime minister came up with to give communities in Northern Ireland a regular say over whatever comes into effect.
The BBC has learned that the draft Brexit deal has a mechanism enabling Northern Ireland to approve or reject the border plans.
This would give the Stormont Assembly the chance to vote on Brexit arrangements four years after the Brexit transition period ends in 2020.
As well as the DUP, Mr Johnson is also trying to secure support from Tory Brexiteers, most of whom are part of the European Research Group.
ERG chairman Steve Baker told reporters after a meeting in Downing Street on Wednesday evening his group “hope [to] be with the prime minister, but there are thousands of people out there who are counting on us not to let them down and we are not going to”.
“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 see that text, we can’t say.”
Former Attorney General Dominic Grieve, who wants any new deal to be put back to the public to approve, said it was not “realistic” for the prime minister to expect Parliament to scrutinise and approve a legal text on Saturday – even if Mr Johnson does manage to finalise the plan with the EU.
“I would normally expect there to be a week at least between a text being laid before Parliament and Parliament voting on such a matter of importance,” the backbench Conservative MP told BBC Radio 4’s Today programme.
The Irish Prime Minister Leo Varadkar has said that if a deal cannot be completed at the two-day summit, European leaders could gather again before the end of the month to continue Brexit talks.
On Wednesday, Mr Johnson likened the Brexit talks to climbing Everest, saying the summit was “not far” but still surrounded by “cloud”, according to MPs he addressed at the 1922 committee of backbenchers.
Mr Johnson faces another deadline on Saturday – the date set out in the so-called Benn Act, which was passed last month by MPs seeking to avoid a no-deal Brexit.
If MPs have not approved a deal – or voted for leaving the EU without one – by Saturday, then Mr Johnson must send a letter to the EU requesting an extension to 31 January 2020 – something he has repeatedly refused to do.
On Wednesday, Brexit Secretary Stephen Barclay confirmed Mr Johnson would write such a letter if no deal was in place by Saturday.
The prime minister’s official spokesman confirmed the government will table a motion for Parliament to sit this Saturday from 09:00 to 14:00 BST.
However, this does not mean the House of Commons will definitely sit on Saturday – the government could table the motion but not push it to a vote.