The draft Withdrawal Agreement was immediately condemned by Theresa May’s critics on the Conservative back benches, as well as her allies in the Democratic Unionist Party (DUP), adding to growing fears that the Commons could reject the deal and send the UK crashing out of the EU.

The Prime Minister will assemble her Cabinet tomorrow afternoon and seek their approval for the draft Withdrawal Agreement. Senior ministers were summoned to Downing Street for individual meetings with Mrs May tonight in a bid to shore up support.

The text is understood to include contentious elements of the ‘backstop’ plan to maintain the status quo along the Irish border after Brexit, including separate regulatory checks for goods moving between Northern Ireland and the rest of the UK.

British Prime Minister Theresa May attends the Nordic Council Sessions 2018 at Stortinget ( Parliament) on October 30, 2018 in Oslo, Norway. (Photo by Rune Hellestad/Getty Images)

It sets out a proposal for the UK to remain under EU customs union rules until a trade deal between London and Brussels can guarantee the free flow of goods across the Irish border, but an ‘exit mechanism’ from the customs backstop is not expected to include a firm end date or give the UK the right to walk away unilaterally.

Brexiteers and DUP MPs seized on leaked details of the draft agreement, with Jacob Rees-Mogg, the leader of the pro-Brexit European Research Group of Tory backbenchers, calling on cabinet to reject the draft as “a failure of the government’s negotiating position and a failure to deliver on Brexit”.

Boris Johnson dismissed the breakthrough as “vassal state stuff” and claimed it “failed to protect our precious Union”.

“For the first time since partition, Dublin will have more say over aspects of government in Northern Ireland than London,” the former foreign secretary added. “Am I going to vote against it? Yes.”

Scottish Conservative MP Ross Thomson suggested Northern Ireland was being “hived off”, and the DUP’s Westminster leader Nigel Dodds said: “If the reports are as we are hearing, then we couldn’t possibly vote for that.”

Cabinet will meet at 2pm tomorrow afternoon, at the same time as ambassadors from the 27 other EU member states gather in Brussels for a briefing on the draft text from chief Brexit negotiator Michel Barnier.

If ministers give their approval, the draft deal could be published as early tomorrow afternoon, with the Prime Minister unveiling the agreement with a Downing Street statement, UK government sources suggested.

However, several cabinet ministers including the Brexit Secretary Dominic Raab are understood to be unhappy with aspects of the Irish border backstop, and the risk that the UK could be prevented from signing its own trade deals if it is held within the EU customs union indefinitely.

Scottish Secretary David Mundell is expected to view the draft agreement tonight. Welcoming the progress, he said it was “encouraging there’s a potential agreement”.

“We need to reflect on that detail and see what’s there and hopefully be in a position to take forward a deal,” he told journalists.

Mr Mundell and the Scottish Conservative leader Ruth Davidson have previously signalled their refusal to accept new trade barriers in the Irish Sea.

In a letter to Prime Minister Theresa May last month, both senior Tories highlighted that “having fought just four years ago to keep our country together, the integrity of our United Kingdom remains the single most important issue for us in these negotiations.”

They said: “Any deal that delivers a differentiated settlement for Northern Ireland beyond the differences that already exist... would undermine the integrity of our UK internal market and this United Kingdom.

“We could not support any deal that creates a border of any kind in the Irish Sea and undermines the Union.”

As well as the UK’s divorce treaty, the draft texts includes an outline declaration of objectives for the future trade and security partnership between London and Brussels, which could include details of EU demands for access to British fishing waters - another contentious issue for Scottish Conservative MPs.

Responding to news of the potential breakthrough on social media, Nicola Sturgeon said: “If the PM’s ‘deal’ satisfies no-one and can’t command a majority, we mustn’t fall for her spin that the UK crashing out of EU without a deal is then inevitable – instead we should take the opportunity to get better options back on the table.”

“Also, though the Withdrawal proposals will get most attention initially, the declaration on future relationship is just as important.

“If that is vague and dodges tough issues, a blindfold Brexit would beckon. It would be deeply irresponsible for parliament to sanction that.”

As news of the draft agreement broke, ministers from devolved administrations were in London for their latest meeting with UK counterparts on post-Brexit power-sharing.

Their meeting in the Cabinet Office, where a reading room was set up tonight for ministers to review the draft deal, broke up half an hour early.

Scottish Government’s Constitutional Relations Secretary Michael Russell issued a call for devolved administrations to be shown the text, but UK government sources said it couldn’t be shared before the Cabinet had seen it.

“It is completely unacceptable that devolved administrations have still not seen the detail of any draft deal,” Mr Russell said following the meeting. “We know no more coming out of tonight’s JMC than we did going in.”

Theresa May braced for Cabinet revolt over Brexit deal