Pro-Brexit Cabinet ministers have been urged to resist any attempts to keep the UK closely tied to Brussels ahead of a crunch meeting at Chequers.
Theresa May is trying to force through a compromise that will secure support from both wings of her Cabinet at the gathering on Friday, but critics called for the Government to maintain its red lines on leaving the single market and customs union.
Brexiteer Cabinet minister Michael Gove insisted that a "united position" would be reached at the away day, ahead of the publication of a White Paper outlining the Government's blueprint for the future relationship with the EU next week.
And he played down speculation that pro-Brexit ministers such as Foreign Secretary Boris Johnson could quit if there was perceived to be backsliding over the break from Brussels.
Tory MP Andrea Jenkyns, who quit as a ministerial aide in order to speak more freely about Brexit, urged Cabinet Brexiteers to stand up for Leave voters - and referred to the jockeying for position to succeed Mrs May.
In a message on Twitter aimed at Mr Johnson, Mr Gove, Penny Mordaunt, Liam Fox, Andrea Leadsom and Chris Grayling, she urged them to "show your steel on Friday".
"We know some of you want to be the future party leader," she said.
Speculation has been mounting that the Prime Minister will set out plans for a soft Brexit in order to overcome the problems at the Irish border and ports across the UK if there were customs or regulatory barriers to trade.
Ms Jenkyns warned the Prime Minister's position would be under threat if she opted for a Norway-style relationship with the EU inside the single market.
"If we don't deliver Brexit, if we're half in and half out, it's going to be catastrophic for the Conservative Party," she told the ConservativeHome website.
"They're not going to trust the party. In our manifesto, all of us, Brexiters and Remainers, we stood on that manifesto that we would deliver Brexit."
She added that "history shows, prime ministers keep their jobs if they keep their promises".
Jacob Rees-Mogg, leader of the influential European Research Group of pro-Brexit Tories, told Mrs May he may not back a Brexit deal which left Leave voters "cheated".
"It's not for me to issue warnings to the Prime Minister. I fully support the Prime Minister, but I cannot guarantee to support a policy that doesn't deliver on the manifesto commitments," he told Channel 4 News.
Insisting that the red lines remained intact Mr Gove said: "The Prime Minister has a clear plan which will ensure that Britain can leave the EU, be outside the single market and customs union, maintain as frictionless access as possible to the European market and also ensure that we take back control of our laws and our borders."
The Environment Secretary told BBC Radio 4's Today programme: "What we are going to do is to make sure that we have a discussion which ensures that we have the right policy for the UK."
He said at the end "I think the Cabinet will agree a united position", adding: "Everyone in Cabinet is an advocate for Brexit, that is government policy."
Asked about the possibility of people quitting as ministers after Friday's showdown, Mr Gove said: "The only departures that I think we will see are more departures from Heathrow when a third runway is built."
The summit at Chequers, the Prime Minister's country residence, is intended to fix the Government's position on key areas of the trade deal it wants with Brussels, but details remain vague on a reported "third way" to manage customs arrangements at ports and the Irish border.
The two original competing options for a post-Brexit deal have split the Cabinet and received the cold shoulder in Brussels.
The new proposal is reported to include many of the elements of the "new customs partnership" model rejected by Brexiteers, which would see the UK effectively collect tariffs on behalf of the EU for goods destined for the bloc.
But Cabinet ministers have been kept in the dark about any new proposal ahead of Friday's meeting.
Asked if he had read the new plan, Mr Gove said: "I'm not going to comment on what I have or haven't seen in advance of Friday's meeting."