Remainers led by Stephen Hammond and Nicky Morgan will be pushing a new amendment to the government's trade bill, which will make it a negotiating objective to "participate after exit day in a customs union with the EU", in the event that a deal on frictionless trade can not be secured by January.
The PM is also facing defeat on a vote to bring forward the summer recess to Thursday this week, bringing the break forward by four days despite the political crisis over Brexit.
The East Antrim MP also said the legal assurance that there will be no post-Brexit customs border between NI and GB would effectively kill off the so called "backstop option", whereby Northern Ireland would remain in the customs union in the event of a "no deal" Brexit.
The Prime Minister was saved from a humiliating reverse by the votes of four Labour Brexiteers - and one now sitting as an independent - who backed the Government in the crucial division. "We have to have a compromise position that enables the country to get an agreement with the European Union", trade minister Liam Fox told BBC radio.
Former minister Anna Soubry (pictured left yesterday) said whips had threatened to trigger a general election if the government had lost the customs union vote.
Frank Field, Kate Hoey, John Mann and Graham Stringer - (plus now suspended Kelvin Hopkins) joined the majority of Tories in the voting lobby to defeat the customs union amendment, "New Clause 18".
But despite signs there could be enough support, MPs voted against it by 307 to 301.
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Labour Brexiteers Frank Field, Belfast-born Kate Hoey, John Mann and Graham Stringer - along with independent Kelvin Hopkins - voted with the Government.
"Yesterday changed that", MP Phillip Lee, who resigned as a junior minister over Brexit earlier this year, told lawmakers.
The EU has been urging since last December for the United Kingdom to agree to a so-called "backstop solution" on Ireland that would see Northern Ireland retain parts of the customs union and the single market necessary to keep the border invisible until a more permanent solution is found.
According to a source cited by the Times, Tory Deputy Chief Whip Chris Pincher told rebel MPs they would be responsible for a vote of no confidence if May's plan was shot down.
"I am not saying there will be no kickback from the European Union; I am sure they will try their usual blackmail tactics". The government won several other votes more comfortably, and the entire bill was then approved by the House of Commons by 318 to 285.
"Very significant defeat of govt tonight on European medicines regulation amendment".
Mr Hollingbery, moving the Government's amendments to the Bill, said: "I recognise that members are seeking reassurance that the government will be transparent about the content of these transitioned agreements and what might need to change to deliver this continuity that we have championed for so long".