In Brussels, negotiators were on the brink of signing off a new legal text for a Brexit agreement, with EU chief negotiator Michel Barnier briefing ambassadors from member states that compromise had been reached on all but one of the issues relating to the future of the Irish border.

But in London, Downing Street played down hopes that a deal would be done in time for today’s gathering of European leaders, who must sign off any agreement so it can be approved by MPs on Saturday and avoid a further delay to the UK’s departure.

There was optimism from leading EU figures, with French president Emmanuel Macron telling a press conference: “It’s our hope and will to be able to endorse an agreement and I hope that agreement will be found in the coming hours.”

European Union and British negotiators have failed to get a breakthrough in the Brexit talks during a frantic all-night session and will continue seeking a compromise on the eve of Thursday's crucial EU summit. (AP Photo/Virginia Mayo)

He was speaking alongside German chancellor Angela Merkel, who described yesterday’s talks as the “final sprint” and said she was “increasingly of the belief” a deal would be done.

But in a brief appearance at a meeting of the Conservative 1922 committee, Boris Johnson told his MPs that negotiations were “not quite at the summit … we are at the Hillary Step” – a reference to the final resting point for climbers just below the peak of Everest.

“The summit is not far but at the moment there is still cloud around the summit,” Mr Johnson said.

Earlier, the Prime Minister won the “full support” of the Cabinet to get a deal at today’s summit, the official Downing Street spokesman said – although Mr Johnson is reported to have compared talks to a scene from the film The Shawshank Redemption, where a prisoner escapes from jail through a sewer.

A motion to allow parliament to sit on Saturday for the first time in 37 years to vote on a Brexit deal was tabled yesterday, but must be moved by the government and voted on by MPs today.

With an agreement not yet signed off, there were suggestions that MPs could be asked to take part in another “indicative vote” on an outline political agreement.

But there were warnings that the government would struggle to put together the votes, with the DUP continuing to withhold their support and the leading Brexiteers also wavering.

The chairman of the European Research Group, Steve Baker – who said on Tuesday that he trusted the Prime Minister – insisted he would need to see the legal text of a deal before any vote on it. “If I am not shown the legal text I shall vote no because I haven’t read it,” Mr Baker warned.

DUP leader Arlene Foster rejected reports that her party had accepted a government climbdown on the issue of consent for the devolved Northern Ireland assembly, which EU leaders had feared would grant a veto on arrangements along the Irish border.

She tweeted: “Discussions continue. Needs to be a sensible deal which unionists and nationalists can support.”

And the DUP MP Sammy Wilson also posted on Twitter that the Good Friday Agreement “requires cross-community consent for all controversial issues passing through the Assembly.

“UK and EU negotiators, who have ad nauseam pontificated about the need to respect the Agreement, have no business interfering in the process for consent as currently set out.”

Mr Johnson held his third meeting in three days with DUP leaders yesterday. A DUP spokesman denied reports that the party was looking for “billions not millions” in addition investment for Northern Ireland in exchange for their support.

Arrangements for the collection of VAT on goods traded through Northern Ireland are understood to be the final sticking point to reaching a deal in Brussels.

Reports suggest that under the terms of a revised agreement, while the region would formally exit the EU customs union along with the rest of the UK as demanded by the DUP, it could de facto remain under the Brussels customs regime, with additional checks on goods crossing the Irish Sea.

Fears over Northern Ireland’s place in the Union saw the DUP collapse an attempt at a Brexit breakthrough in December 2017. The issue was also a red line for Scottish Conservatives under Ruth Davidson.

Former Scottish secretary David Mundell backed away from previous opposition to a separate Brexit settlement for Northern Ireland, which Scottish Tories had feared would be seized upon by the SNP as a blueprint for an independent Scotland.

Mr Mundell said in a TV interview yesterday: “People voted three and a half years ago to leave the EU; we haven’t left the EU. The issues which then come forward to be discussed are just not issues that resonate with the public.

“In my constituency, the backstop is something you keep the door open with. It is not something people are fixated with the way some parliamentary colleagues are.”

Mr Mundell said he wanted to see the reaction to the legal text in Northern Ireland, but added: “What I don’t want to do is politicise Northern Ireland in a nationalist way … Northern Ireland is different.”

Negotiations continued throughout Wednesday after running until 1:30am. Mr Johnson spoke to Irish prime minister Leo Varadkar by phone in the morning, with the Taoiseach also giving an upbeat assessment of talks.

However, Mr Varadkar suggested it could take until the end of the month to finalise a deal, suggesting a further EU summit could be held in October, shortly before the Brexit deadline.

“I do think we were making progress but there are issues to be resolved, and hopefully that can be done today, allowing us to ratify the agreement tomorrow and the House of Commons to do it on Saturday,” he said.

Labour leader Jeremy Corbyn said he was “deeply concerned” about the negotiations and ruled out his party backing a deal under Mr Johnson’s terms.

Brexit deal in the balance as Boris Johnson faces crucial EU summit