THERESA May has been accused by Labour of jeopardising the cross-party talks as they enter their crucial phase after details of the key terms of a possible agreement were leaked to the Press.

John McDonnell, the Shadow Chancellor, who has been taking part in the negotiations to find a compromise agreement, accused the Prime Minister of acting in “bad faith”.

Asked if he trusted Mrs May, he replied: “No, sorry, not after this weekend when she has blown the confidentiality we had. She has jeopardised the negotiations for her own personal protection.”

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The talks, which are due to resume on Tuesday, are now said to be centred on agreeing a comprehensive but temporary customs arrangement with the EU lasting until the next General Election, scheduled for 2022; an alignment with a wider range of EU single market regulations on goods and enshrining in law that the UK will mirror all EU legislation on workers’ rights.

Mr McDonnell said Labour wanted the Tories to put party politics aside and wanted to get a deal done "as soon as possible" but it needed guarantees that an agreement would not be "ripped up" by a future Conservative leader.

"We are negotiating with Theresa May's team as requested,” he told BBC1’s Andrew Marr Show. “Whilst we're doing that - and we think we're gaining an understanding of our different positions and where we can reach some compromise - in the wings, if you like, are all the leadership candidates virtually threatening to tear up whatever deal that we do.

"So, we're dealing with a very unstable government and, let me just use this analogy, it's[like] trying to enter into a contract with a company that's going into administration and the people who are going to take over are not willing to fulfil that contract. We can't negotiate like that," he added.

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His colleague Tom Watson, the deputy leader, also introduced a note of caution given rising optimism at Westminster that the two parties were on the brink of a deal. He told BBC 5 Live's Pienaar's Politics: "I don’t think we should give false hope on this; it’s going to be very difficult to find a negotiated settlement.”

Meanwhile, the PM made a direct appeal to Jeremy Corbyn to seal a deal this week.

In a Sunday newspaper article, she wrote: "To the Leader of the Opposition, I say this: let's listen to what the voters said in the elections and put our differences aside for a moment. Let's do a deal.”

Mrs May went on: “I negotiated with the EU what I believe is a very good deal for the UK; a deal which allows us to genuinely take back control of our money and our laws.

"The free movement of people will end, giving us control of our own borders for the first time in decades.

"However, I could not persuade enough of my colleagues to vote for the Withdrawal Agreement and, regrettably, I have to accept there is no sign of that position changing."

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The PM also issued a rallying cry to MPs urging them to support the cross-party efforts to "break the deadlock" and get a Brexit deal through the Commons.

But more than 100 opposition MPs from five parties have written to her and the Labour leader, making clear they will not support a "Westminster stitch-up" and would vote against a customs union proposal unless it was put to a referendum.

Ruth Davidson, the Scottish Conservative leader, who has now returned to the political fray following six months’ maternity leave, argued that that the public positions of the two main parties on Brexit were not that far apart. “There is a deal to be done,” declared Ms Davidson, adding that the answer lay “somewhere in middle” between the Leave and Remain arguments.

Rory Stewart, the new International Development Secretary, said the Conservative Party would have to endure some “short-term pain” to get through Brexit.

"This is the most tortuous, torrid, painful time in British politics since the Second World War,” declared the Borders MP. "And that is because Brexit is probably the most divisive issue Britain has faced since the Second World War."

The former Black Watch officer said the "genius" of the Tory Party was its ability to have room for a wide range of voices and opinions but he accepted that it would have to endure a painful process to get a Brexit deal.

“To get Brexit done and to move this country on is worth an enormous amount…we may have to take some short-term pain to do that.

"But it has got to be the right thing to do because let's take the issues that really matter to me in my new job in international development; climate, for example. That is something where literally, quite literally, the planet is at stake."

Mr Stewart also warned that if the Tories tried to "outdo" Nigel Farage, then it could lose four million Conservative Remain-supporting voters.

"We've got to be a broad party. We've got to be able to stretch all the way from Ken Clarke right the way through to Jacob Rees-Mogg."

However, underlying Mrs May’s precarious position following the disastrous local election results in England was a call from Iain Duncan Smith, the former Tory leader, who described her as a “caretaker PM” and that she should now go.

Mr Farage, leader of the new Brexit Party, challenged Mr Corbyn to a debate ahead of the European elections, warning a deal between Labour and the Tories would be the "final betrayal".

He told Sky News’s Sophy Ridge on Sunday: "There are five million voters out there, Labour voters, who voted to leave, particularly in the Midlands, the north, and south Wales.

"I would love between now and polling to have a debate with Jeremy Corbyn about this because people are very confused about what Labour are standing for."

Elsewhere, Jon Ashworth, the Shadow Health Secretary, said the Government was trying to re-dress their customs union offer and had not really shifted its position.

"The key thing is the Government want to be able to do their own trade deals and my concern is that if we have a trade deal with the United States, for example, that could mean Trump's America and big private healthcare corporations getting their hands on NHS contracts."

"I'm not prepared to countenance that: it's why we need instead a permanent and comprehensive customs union arrangement where we do our trade deals as part of the European Union."