Theresa May’s ability to deliver Brexit is in doubt amid a growing threat to her leadership and concerns over whether she still has the political clout to govern.

Ms May must this week renew the drive to push her EU withdrawal Bill through the House of Commons, with Tory MPs who backed Remain in the referendum threatening to join forces with Jeremy Corbyn’s party to impose changes.

But Labour and other opposition parties warned that Ms May no longer has enough authority over the Conservatives to secure the Bill’s passage, after it emerged some 40 Tory MPs may now be willing to sign a letter of no confidence in her and a note emerged in which two cabinet ministers appear to direct her Brexit policy.

It also follows reports that the EU is preparing for the collapse of Brexit talks and of her Government, which is torn between trying to give more ground in Brexit talks to achieve progress while also maintaining the support of Eurosceptic Tories who want no further compromise.

Labour’s Shadow Brexit Secretary Sir Keir Starmer wrote a letter to the Prime Minister on Monday raising concerns that she no longer has the influence over her own party to deliver key facets of a successful Brexit – in particular, securing a transition period to smooth the withdrawal.

The letter said: “Over recent weeks, it has become increasingly clear that you alone do not have the authority to deliver a transitional deal with Europe and to take the necessary steps to protect jobs and the economy.”

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The letter urged the Government to work with Labour, accepting its amendments to her Bill to come to an agreed position in the “national interest”.

The most difficult Commons battle over her EU withdrawal Bill will not occur until December, after the Budget, but even if it clears its stages in the lower House, Liberal Democrat Leader Sir Vince Cable has told The Independent it will be amended in the Lords by a coalition of opposition and crossbench peers and Tory rebels.

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Sir Keir’s letter went on to point out how cabinet ministers, including Foreign Secretary Boris Johnson and International Trade Secretary Liam Fox, had appeared to make statements that contradicted her Brexit plans as set out in a major speech in Florence earlier in the year.

Reports also emerged at the weekend that Mr Johnson and fellow Brexiteer Michael Gove, the Environment Secretary, wrote a secret memo to the Prime Minister setting out how a transition should occur and indicating that she should be “underlining your resolve” over withdrawal.

The SNP’s Stephen Gethins 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 … goes to show that they think they can say and do as they please, knowing fine well Theresa May is powerless to act.

“Theresa May is Prime Minister only by title.”

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A torrid 10 days has seen Ms May lose two cabinet ministers, Sir Michael Fallon and Priti Patel, while her deputy Damian Green is still under investigation over allegations of inappropriate behaviour which he denies, and Mr Johnson is defending himself against calls to resign over a diplomatic gaffe relating to British woman Nazanin Zaghari-Ratcliffe, who is imprisoned in Iran.

Amid the turmoil the number of MPs willing to put their name to a letter of no confidence is reported to have risen to 40, just eight short of the number needed to trigger a vote on Ms May’s future.

A senior Tory MP told The Independent: “Patience is wearing very thin and in some cases, it has snapped.”

But Brexit Secretary David Davis said he is “quite certain” Ms May will remain Prime Minister at least throughout Brexit, dismissing the Government’s recent series of crises as “flurries” and adding that all governments have “issues that come up and go”.

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The EU’s chief negotiator Michel Barnier confirmed reports that Brussels is planning for Brexit talks to collapse, though he said it is not the preferred option.

On other reports of EU leaders actively planning for a situation where Ms May is not Prime Minister, Mr Davis said: “I know it was in the papers, I would be very surprised if they really are planning that.