A government source has told the BBC there will be “no deal tonight”, as officials continue to work on the technical details in Brussels.
The UK and EU had been hoping to sign off a revised Brexit deal before Thursday’s crunch EU council meeting.
The BBC’s Laura Kuenssberg said it was not now clear whether there would be a deal at all this week.
Boris Johnson has been trying to get Tory Brexiteers and the DUP to back his revised plan for Northern Ireland.
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, whose support could be vital if Parliament is to approve any agreement. were ready to sign up or not.
Likening the Brexit talks to climbing Everest, the prime minister said the summit was “not far” but still surrounded by “cloud”.
Tory MP Steve Baker, chairman of the pro-Brexit European Research Group of MPs, whose support in Parliament could also be vital, spoke to reporters after a meeting in Downing Street on Wednesday evening.
“We hope we will 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,” he said.
“We are just really wishing the prime minister well and hoping he has total success. 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.”
‘We are working’
The EU’s chief negotiator Michel Barnier has, meanwhile, been briefing EU ambassadors, ahead of Thursday’s summit.
The meeting was originally due to take place at lunch time but was put back twice.
Asked afterwards, whether there was a deal, Mr Barnier said: “We are working, we are working.”
Boris 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.
The prime minister’s official spokesman has confirmed the government will table a motion for Parliament to sit this Saturday from 09:00 to 14:00 BST.
That motion would be considered on Thursday.
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.
Analysis: Nick Eardley, BBC political correspondent
Conservative Brexiteers want to believe Boris Johnson – they trust him, but they have not seen the legal text of a deal so they are not saying definitively “yes, we will vote for it.”
The most important players are the DUP.
We think they are seeing the PM again tonight.
Those conversations could be crucial.
If the DUP fall in behind him then it seems that Boris Johnson would have the numbers to get over the line.
But if they don’t, it is very hard to see how he could do it.