Theresa May has thrown her weight behind a parliamentary move to win her more Brexit negotiating power by rallying MPs behind an alternative to the controversial backstop.
Conservative chairman Brandon Lewis announced that Tory MPs will be whipped to support an amendment tabled by senior backbencher Sir Graham Brady in a series of votes on Tuesday evening.
And Tory backbenchers Andrew Murrison and John Baron withdrew their rival amendments, which would have required the scrapping or time-limiting of the backstop, which is designed to avoid the need for a hard border in Ireland following Brexit.
But the dramatic move was far from guaranteed to deliver a majority for the plan, after it failed to win the support of the influential European Research Group of Conservative eurosceptics.
The amendment tabled by Sir Graham, the chair of the Tory backbench 1922 Committee states that Parliament would be willing to support the Withdrawal Agreement reached with the EU last November, so long as the backstop is replaced with "alternative arrangements" to avoid a hard border.
The Tory grandee said that he hoped that House of Commons backing would give Mrs May "enormous firepower" when she returns to Brussels to seek concessions on her Brexit deal.
But the chairman of the European Research Group, Jacob Rees-Mogg, said that its members, thought to number at least 60, will not support the scheme when it comes before the Commons on Tuesday.
Mrs May addressed MPs in Westminster on the eve of a day of high drama, when MPs will debate a range of rival amendments, including calls to block a no-deal departure or delay Brexit from its scheduled date of March 29.
Emerging from a meeting of the ERG just moments before the PM's arrival, Mr Rees-Mogg said the group would not support any amendment, but would decide on the day whether to abstain or vote against.
MPs in the ERG were concerned that the Brady amendment did not spell out what would replace the backstop and did not specify that changes to the text of the Withdrawal Agreement were required, he said.
Mr Lewis said that the Brady amendment would "allow the Prime Minister to give a very clear message around what Parliament wants, where the party is".
He said: "I would hope the ERG, when they look at this and actually look through the detail of what this gives the PM tomorrow, (see) it is about giving a message to Europe about what can go through Parliament in terms of dealing with the backstop issue and why that matters."
Sammy Wilson of the DUP, whose 10 MPs prop up the minority Conservative administration, said the PM should "exploit the cracks which are emerging in the illogical position of the EU and the Irish".
Insisting the UK must hold to the date of March 29 to maintain pressure on the EU and Ireland, Mr Wilson said: "Now is the time for the Government to be tough and to face down the stubbornness of Dublin and Brussels."
There was no sign in Brussels of any appetite to reopen the Withdrawal Agreement.
Chief EU negotiator Michel Barnier's deputy Sabine Weyand warned there was now a "high risk" of the UK crashing out by accident, as it was "quite a challenge" to see how a majority can be constructed at Westminster.
And European Commission vice-president Jyrki Kateinen said there was "no reason to give any concessions" to the UK and there was "not much room for manoeuvre" on the backstop.
It would be "stupid" for the EU to make concessions putting the remaining 27 members at a disadvantage simply to secure a deal, he said.
Downing Street said the PM remains committed to quitting the EU on March 29 and will take her plan back to the Commons for a second "meaningful vote" as soon as possible after Tuesday's debate.
"The Prime Minister is absolutely committed to leaving the EU with a deal, but clearly if we are to obtain parliamentary support for that deal some changes are going to have to be made," the PM's spokesman said.
The developments came as major supermarkets and fast-food chains warned that a no-deal Brexit will force up prices and reduce the choice and quality of produce for shoppers.
In a joint letter, bosses of firms including Sainsbury, Asda, KFC and McDonald's said: "We anticipate significant risks to maintaining the choice, quality and durability of food that our customers have come to expect in our stores, and there will be inevitable pressure on food prices from higher transport costs, currency devaluation and tariffs.
"We are therefore asking you to work with your colleagues in Parliament urgently to find a solution that avoids the shock of a no-deal Brexit on 29 March and removes these risks for UK consumers."
Meanwhile, former MI5 chief Baroness Manningham-Buller warned that no-deal Brexit should be "avoided at all costs" because security threats, from terrorism to Russian interference, were best dealt with "in a European context".
"If we leave without a deal we are going to be less safe," she told BBC Radio 4's World At One.
There was no announcement from Labour over whether it will officially back a cross-party amendment put forward by the party's Yvette Cooper and Conservative Nick Boles aimed at stopping a no-deal exit and delaying the date of Brexit until the end of the year.
Mr Boles said the plan has "a great deal of support among ministers and indeed Cabinet ministers" and they were pressing for a free vote.
"This is the last chance for Parliament, this is probably the only opportunity that Parliament is going to have to intervene in this process, to take control," he said.
The Federation of Small Businesses gave its backing to the Cooper amendment, which it said would "help provide the space for other ways forward to be explored".
"A stable, predictable economic and political environment is a prerequisite for small business growth and the jobs that this sustains," said FSB chair Mike Cherry.
"With small business confidence already at its lowest since 2011, a no-deal Brexit on March 29 would put this at risk."