Forty Conservative MPs are reportedly prepared to sign a letter of no confidence in Theresa May, amid claims she is being held "hostage" by two members of her top team.
The disclosure in the Sunday Times will pile the pressure on the Prime Minister as she enters a crucial period in the Brexit negotiations and comes as she attempts to steady the ship following the resignations of two Cabinet ministers.
The total is eight short of the number needed to trigger a leadership contest under Conservative party rules.
Meanwhile, the Mail on Sunday has obtained what it says is a leaked letter from Boris Johnson and Michael Gove, in which the two Vote Leave figureheads tell the PM how to deliver Brexit.
A senior Government source told the newspaper the memo represented a "soft coup" and showed Mrs May was "their Downing Street hostage".
Mr Johnson and Mr Gove have not spoken about the emergence of the document so far, while Number 10 said it did not comment on leaks.
According to the Mail, Mr Johnson and Mr Gove demand in the note that the Brexit transition must end on 30 June 2021 and call on the PM to ensure the Cabinet falls behind the Brexit plan by "clarifying their minds" and making sure they "internalise the logic".
The pair also appear to take a thinly veiled shot at the Remain-backing Chancellor Philip Hammond for lacking the "sufficient energy" in prepare for Britain's post-Brexit future.
Titled EU Exit – Next Steps and marked "For your and Gavin's eyes only", the letter states: "Your approach is governed by sensible pragmatism. That does not in any way dilute our ambition to be a fully independent self-governing country by the time of the next election.
"If we are to counter those who wish to frustrate that end, there are ways of underlining your resolve.
"We are profoundly worried that in some parts of Government the current preparations are not proceeding with anything like sufficient energy.
"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…"
Labour leader Jeremy Corbyn has piled in, calling on Mrs May to "govern or go".
He writes in an article in the Sunday Times: "Continuing uncertainty about the Government's approach to Brexit is now the biggest risk facing our country.
"The Prime Minister must end the confusion, take on the 'no-deal' extremists in her Government and back a jobs first Brexit for Britain."