The future of Boris Johnson’s Brexit deal hangs in the balance as EU officials say the outcome of talks should be known by the end of the day.
The EU’s Donald Tusk said he would have “bet” on a deal 24 hours ago, but “doubts” had appeared on the UK side.
The PM is trying to get Tory Brexiteers and Democratic Unionists on board for his revised plan for Northern Ireland.
Likening talks to climbing Everest, Mr Johnson said the summit was “not far” but still surrounded by “cloud”.
He is in a race against time to get a deal before Thursday’s crucial EU council meeting.
The BBC’s political editor Laura Kuenssberg said she understood the issues between the UK, EU and Ireland were “pretty much sorted”, but it was still not clear whether the DUP were ready to sign up or not.
Mr Johnson has been updating his cabinet on the state of the negotiations after further talks with the Northern Irish DUP, whose support could be vital if Parliament is to approve any agreement.
The BBC understands ministers were not briefed on the full contents of the proposed deal because the situation in Brussels is still fluid.
The PM has also briefly addressed a meeting of Conservative MPs, comparing the current position to Edmund Hillary’s ascent of Mount Everest in 1953.
The EU’s chief negotiator Michel Barnier was due to brief EU ministers at lunch time but this meeting has been put back twice – and will not now take place until 18:00 BST.
The issue of the Irish border – and how to handle the flow of goods and people across it once it becomes the border between the UK and the EU after Brexit – has long been a sticking point in the negotiations.
The border is also a matter of great political, security and diplomatic sensitivity in Ireland.
The backstop – the solution to border issues agreed by Theresa May – proved unpalatable to many MPs so Mr Johnson has come up with new proposals to dispense with it.
However, they would see Northern Ireland treated differently from the rest of the UK – something the DUP, among others, has great concerns about.
The BBC’s Brussels reporter, Adam Fleming, said that during the course of Wednesday there had been a shift in emphasis in the Brussels talks away from issues around customs towards matters surrounding 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 DUP is understood to be most concerned about this issue too. The party also fears the creation of a “customs border” in the Irish Sea, which would require checks on goods between the rest of the UK and Northern Ireland.
The DUP held their latest round of talks in Downing Street on Wednesday morning. After a 90-minute meeting on Tuesday, they said “it would be fair to indicate gaps remain and further work is required”.
Party leader Arlene Foster dismissed suggestions that their concerns had since been allayed.
The UK is due to leave the EU at 23:00 GMT on 31 October and Mr Johnson has repeatedly insisted this will happen, regardless of whether there is a deal or not.
One senior EU diplomat has told journalists in Brussels it is now too late for EU leaders to formally approve a revised Brexit deal at the summit.
They said the most they could do was give a provisional thumbs-up – “a political yes” – to whatever emerges from the talks pending the release of the final legal text.
But Mr Tusk suggested an agreement was still possible within the next few hours.
“It is still undergoing changes and the basic foundations of this agreement are ready and theoretically we could accept a deal tomorrow.” he told the TVN 24 News Channel.
Thursday’s summit is crucial because under legislation passed last month – the Benn Act – Mr Johnson is compelled to ask the EU for a delay to Brexit if he does not get a new deal approved by MPs by Saturday.
Brexit Secretary Stephen Barclay told MPs on Wednesday that Mr Johnson “will comply with the law” regarding the terms of any further extension.
Irish Taoiseach Leo Varadkar, who earlier spoke to Mr Johnson, also suggested there was still “more time” for a breakthrough.
“There is a pathway to a possible deal but there are many issues that still need to be fully resolved, particularly around the consent mechanism and also some issues around customs and VAT,” he said at an agri-food event in Dublin.
“I do think we are making progress, but there are issues yet to be resolved.
If Mr Johnson gets an agreement, he will ask MPs to back it and agree the next steps in an emergency sitting of Parliament on Saturday.
No 10 has confirmed the government will table a motion in the Commons on Thursday which, if approved by MPs, would pave the way for the first weekend session since 1982.
However, MPs may not be asked to sanction the extra sitting if there is not a successful conclusion to the Brexit talks.
Earlier, former Brexit Secretary David Davis said the support of Tory Eurosceptics could not be taken for granted and MPs would subject any agreement to “two or three key tests” – including whether it compromised the future of the United Kingdom.
“Quite a lot of Tory MPs will take their line from what the DUP say,” he told BBC Radio 4’s Today. “If the DUP say this is intolerable, that will be quite important.”
Meanwhile, a cross-party group of MPs has arrived in Brussels to make the case for another extension to the Brexit process even if Parliament approves a deal on Saturday.
One of the group, ex-Conservative minister Dominic Grieve, told Sky News the PM was trying to “bamboozle” everyone into thinking the process of ratifying any agreement in UK law could be concluded by 31 October when it “plainly cannot”.