THE prospect of a Brexit breakthrough has been cast into serious doubt after the Labour leadership said it could not trust Theresa May, accusing her of “bad faith” over a leak to the Press of the key points of a possible compromise deal.

And in a separate development Rory Stewart, the new International Development Secretary, warned the Conservatives that they would “lose Scotland” if they chose to go down a hard Brexit route.

As a resumption of the cross-party talks on Tuesday is looking increasingly like a make-or-break moment, John McDonnell, who is part of Labour’s negotiating team, hit out at the Prime Minister.

Asked bluntly if he trusted her, the Shadow Chancellor 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.”

Downing St declined to comment on the claim with sources making clear that the Government was fully focused on “getting a deal over the line that is acceptable to both sides”.

The talks, which Mrs May said she wanted to bring to a conclusion this week, are now said to be centred on three key issues: 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.

Following the Tories’ disastrous showing in the English local elections when the party lost more than 1,300 seats, the PM used a Sunday newspaper article to make a direct appeal to the Labour leader, writing: “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.”

But Mr McDonnell stressed getting a deal needed guarantees that an agreement would not be "ripped up" by a future Conservative leader.

He told the BBC’s Andrew Marr Show: “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."

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

Mr Stewart echoed the point that a deal could be done, noting: “A lot of this rests on, to be honest, one man: whether Jeremy Corbyn really wants to deliver a Brexit deal.

"But if he wants to do it, it will be actually surprisingly easy to do because our positions are very, very close," he told Sky’s Sophy Ridge on Sunday.

However, the former Black Watch officer acknowledged that securing a compromise deal would cause the Tories 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.

“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.”

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

"We'd lose young people, we'd lose Scotland, we'd lose London and we'd lose a lot of the most energetic parts of this country. 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," he added.

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