THERESA May is resisting pressure to sack UK Government ministers and aides involved in a bid to bounce her into a hard Brexit.

Between 30 and 40 Conservatives, believed to include Owen Paterson, the former Northern Ireland Secretary, have signed a letter spelling out their Brexit demands, including a transition period in which Britain would be able to sign free trade deals and stop paying money to Brussels.

The letter, circulated among members of the Conservative European Research Group [ERG] of Brexit-backing MPs by Change Britain, also states that remaining in the single market during a transition would be an "historic mistake".

Public money is used to fund the ERG, which was formerly chaired by Brexit minister Steve Baker, but its full membership list is not freely available.

Suella Fernandes, who now chairs the group and is an aide to Chancellor Philip Hammond, insisted the letter was supportive of the Government.

But Stephen Hammond, the former Conservative minister, said the letter was "entirely at odds" with government policy and supporters on the Government's payroll must quit or be sacked.

Nicky Morgan, the pro-Remain former Education Secretary, said the letter undermined the Government and the Tory party.

“It is unacceptable for a group within the Conservative parliamentary party to issue demands. [The letter] says we demand that these things are in the withdrawal agreement, which seeks to undermine the UK’s negotiations within the EU. It’s fine for backbenchers, like me for example, to say what they would like, but this letter goes a little further: it demands certain things and it does go beyond government policy,” she added.

The Prime Minister is preparing to make a keynote speech on Brexit when she is expected to set out her desire for a “no cliff-edge” transition deal. Within days, however, she will be addressing the Conservative Party conference when she is expected to set out a much tougher vision for Britain outside the EU.

The Liberal Democrats claimed the Change Britain letter, expected to be published this Sunday, showed the Conservatives were in "meltdown" and urged Mrs May to "assert her authority".

Alistair Carmichael, the party’s Chief Whip, said: "That an aide to the Chancellor and a Brexit minister can organise a revolt against the apparent official policy of their own government suggests that this Conservative administration is in some kind of meltdown.

"It is very hard to see how Suella Fernandes can remain as an aide to the very Chancellor she is apparently undermining.

"Surely now, in the interests of presenting a united front in negotiations on behalf of the United Kingdom, Theresa May must assert her authority. It is time she acted like a prime minister."

But, when asked, the PM’s spokeswoman said Mrs May was not planning to take any action against ministers or aides said to be involved in the letter.