It's been revealed ex-Tory ministers Ken Clarke and Anna Soubry rebelled against the government by voting against the motion to disagree with Lords amendment, created to give Parliament a vote to prevent a "no deal" Brexit.
May's fragile government will be trying to defeat a rebellion by pro-EU lawmakers and reverse changes to its key piece of Brexit legislation as the matter comes before the House of Commons for two days of debate.
In a painful blow the the PM, Remain-supporting MP Philip Lee quit as justice minister this morning, saying he could not support "how our country's exit from the European Union looks set to be delivered".
"If they throw it out, well they throw it out", he said. "It is, however, irresponsible to proceed as we are".
He said giving Parliament power to direct the government's hand in talks would be "an unconstitutional shift which risks undermining our negotiations with the European Union".
A Downing Street source said: "We will get a good Brexit deal that works for everybody in the UK".
Lee, who voted for Britain to remain in the E.U.in the 2016 referendum, said in a statement he was "incredibly sad" to resign but did so in order to vote against the government's position on a key amendment to the bill.
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And to avoid a sudden "cliff edge" on Brexit day, 29 March 2019, it would also convert existing European Union law into United Kingdom law so the government and Parliament can decide at a later date which bits they want to keep or change.
"We will wait and see the details of this concession and will hold ministers to account to ensure it lives up to the promises they have made to parliament".
Prior to the vote, Tory ministers and whips were engaged in frantic negotiations to prevent a damaging defeat over parliament's ability to block a no-deal Brexit, which came right down to the wire even as the debate carried on.
In the tense atmosphere where it was not clear which way the vote would go, the government secured its victory only after offering concessions to one of the leaders of a group of Conservative lawmakers who were threatening to vote against May.
Ministers have told rebel Conservatives they will agree to seek House of Commons approval for their course of action if no political agreement has been reached on the Withdrawal Agreement agreed with Brussels by November 30, which the original amendment had demanded.
Ms. May's minority government relies on the support of Northern Ireland's Democratic Unionist Party for a slender working majority in the 650-member Commons.