Ministers and aides involved in a Tory bid to bounce the Prime Minister into a "extreme" Brexit must be sacked, Theresa May has been urged.
Between 30 and 40 Conservatives signed a letter spelling out Brexit demands, including a transition period in which Britain will 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 a "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 Tory former minister Stephen Hammond said the missive was "entirely at odds" with government policy and supporters on the Government's payroll must quit or be sacked.
He told The Times: "The European Research Group letter is an unacceptable attempt to hinder negotiations and jeopardise the Government.
"It is entirely at odds with stated policy, which colleagues should be supporting, not undermining.
"It would therefore be completely unacceptable for any minister or parliamentary private secretary to support it or encourage others to sign. They should either sign and resign."
The Liberal Democrats claimed it showed the Conservatives were in "meltdown" and said Mrs May must "assert her authority".
Former cabinet minister Alistair Carmichael 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."
Ministers have made clear that Britain will quit the single market and customs union in March 2019, but have admitted the UK would not be able to implement free trade agreements during a proposed transitional customs deal.
The letter states: "Continued membership of the single market, even as part of a transitional arrangement, would quite simply mean EU membership by another name - and we cannot allow our country to be kept in the EU by stealth.
"The Government must respect the will of the British people, and that means leaving the single market at the same time as we leave the EU."
Ms Fernandes refused to disclose the membership of the ERG, which is believed to include ministers.
She told Channel 4 News: "It is funded by public money and it has been for many years.
"I'm not going to say how many government ministers are in it because it is essentially a group of Members of Parliament. It's not a secretive society."