THERESA May’s authority has taken another battering after the leak of a letter from two pro-Brexit cabinet ministers telling her how to do her job.

Boris Johnson and Michael Gove were accused of mounting a “soft coup” after issuing instructions to the Prime Minister on how to handle the UK’s withdrawal from Europe.

The SNP said the Foreign and Environment secretaries were holding Mrs May “to ransom” and that she was now “Prime Minister only by title”.

It was also reported that 40 MPs - eight short of the threshold for triggering a confidence vote - had agreed to sign a letter calling on Mrs May to stand down.

The leak of the Gove-Johnson letter to a Sunday newspaper coincided with the EU’s chief negotiator confirming Brussels was contingency planning for the Brexit talks to collapse.

Michel Barnier told a French newspaper: “It’s not my option, but it’s a possibility. Everyone needs to plan for it, member states and businesses alike.

“We too are preparing for it technically. A failure of the negotiations would have consequences on multiple domains.”

His remarks came just 48 hours after UK Brexit Secretary David Davis insisted the negotiations had narrowed to a “few outstanding, albeit important, issues”.

The EU has given the UK until the end of the month to raise its £20bn offer on the Brexit divorce bill, otherwise there will be no advance to deeper trade talks.

Mr Davis told Sky News yesterday that British taxpayers "would not want me to just come along and just give away billions of pounds", adding: "We've been very, very careful, and it's taking time and we will take our time to get to the right answer."

As to Mrs May’s shelf-life, he added: “The Prime Minister will be here right through Brexit... she'll my boss for that - I'm quite certain of it."

The secret letter from Mr Johnson and Mr Gove to Mrs May, titled EU Exit - Next Steps, was marked “For your and Gavin’s eyes only”, a reference to her chief of staff, Gavin Barwell.

It said Brexit transition arrangements must end on 30 June 2021, and urged the Prime Minister to ensure members of her top team got behind their plans by “clarifying their minds” and called for them to “internalise the logic”.

It included a veiled attack on Chancellor Philip Hammond, who backed Remain, saying some parts of the Government lacked “sufficient energy” in preparing for Brexit.

It said: "We have heard it argued by some that we cannot start preparations on the basis of 'No Deal' because that would undermine our obligation of 'sincere co-operation' with the EU. “If taken seriously, that would leave us over a barrel in 2021. We all want you to push your agenda forward with confidence and have your Government articulate the following..."

A senior Government source told the newspaper the authors had conducted a “soft coup” and described Mrs May as “their Downing Street hostage”.

Appearing on the BBC yesterdaya, Mr Gove did not deny writing the letter but refused to comment on it as its was private, and said he would not block Mrs May if she decided to hand over more money to Brussels for a good exit deal.

Stephen Gethins, the SNP’s foreign affairs spokesman, said: “If it wasn't clear before, it is now - Theresa May has lost all authority and credibility in government.

"The revelation that leading Brexiteers Boris Johnson and Michael Gove are now brazenly able to dictate their hard Brexit demands over a month ago, goes to show that they think they can say and do as they please, knowing fine well Theresa May is powerless to act."

The main legislation underpinning Brexit, the EU (Withdrawal Bill), returns to the Commons tomorrow, with opposition and rebel Tory MPs planning multiple ambushes to amend it.

Mrs May last week warned rebels not to impede its progress and promised the precise date for Brexit - 11pm on 29 March 2019 - would be included on the face of the Bill.

However, in a sign of the Prime Minister’s inability to get back on the front front, her attempt to reassert her authority was read as yet another sign of her weakness and vulnerability.