In a day of drama, May's position seemed suddenly weaker when junior justice minister Phillip Lee, who has always been critical of the government's Brexit strategy, resigned and said he would vote against the government.
A minister long critical of the government's handling of Brexit resigned earlier in protest at what he called its "wish to limit" the role of lawmakers in the process.
May is resisting changes approved by the House of Lords that would soften Britain's exit from the European Union, because she says they will weaken the government's negotiating position.
Lee said "the people, economy and culture of my constituency will be affected negatively" by Britain's European Union departure, and it is "irresponsible to proceed as we are".
Despite depending on the votes of the 10 DUP MPs for her precarious Commons majority, there were signs of cautious optimism among ministers that they would get the numbers to see off the revolt.
North Down MP Lady Sylvia Hermon said that, in the absence of a functioning assembly in Northern Ireland, it was critically important that MPs had the opportunity to fully examine and discuss the key amendments relating to no hard border and to north/south cooperation.
Opening debate on the flagship EU Withdrawal Bill in the House of Commons, Mr Davis warned that the "cumulative effect" of a series of Lords amendments would "make it impossible to deliver the smooth and orderly exit we want".
That clause - drafted by Grieve - basically hands a lot of power to Parliament if no deal has been agreed by the end of November.
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.
Brexit Minister Steve Baker said the Government would "look very carefully" at the amendment, which is being tabled by former Cabinet Ministers Oliver Letwin and Nicky Morgan.
His resignation didn't prompt an immediate response from the government, but at the eleventh hour, ministers evidently feared a Commons defeat. "We have no doubt of what was going on".
Rebels said May had assured them the government would accept two parts of Grieve's amendment; and immediately open talks on a third, "part c", which would allow MPs to direct the government, if no deal is reached by February next year. If some of its leave-supporting lawmakers choose to vote against the amendment, the government could avoid defeat altogether.
Her fellow Conservative backbencher Stephen Hammond said: "Parliament must be able to have its say in a "no deal" situation".
If enough of them side with the opposition, the prime minister could lose some key votes.
However, the compromise is only likely to delay a full-blown showdown on the customs union until trade legislation returns to the Commons in July.
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