In a night of high drama, rebel ringleader Dominic Grieve told the Government it was "too late" as ministers made last-minute concessions in an attempt to head off the revolt.
Given that the Prime Minister's working majority is only 13, she will lose - if those rebels stand firm, amid heavy arm-twisting from Tory whips not to give her "a bloody nose".
The remarks came after London and Brussels announced a much-anticipated breakthrough in Brexit talks, which paved the way for the two sides to shift to the second phase of talks, related to future trade relations.
The government sought to stave off the rebellion on its own benches in the hours before the debate, first with a statement from Brexit secretary David Davis promising a vote in parliament "as soon as possible" after an agreement with the European Union is reached.
Members of Parliament voted Wednesday for an amendment to the bill meaning lawmakers must approve the final deal with the European Union before withdrawal begins.
It passed by 309 votes to 305, with a twelfth Conservative MP effectively abstaining by voting in both camps.
He said his amendment would not stop Brexit but the vote provoked a furious backlash from Leavers.
Hours before the vote, Brexit Secretary David Davis promised MPs that no withdrawal agreement would be implemented until a vote in parliament.
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Members of Parliament are debating the EU withdrawal bill, which will repeal the 1972 legislation binding Britain to the EU and copy existing EU law into domestic law to ensure legal continuity after "Exit Day" on March 29, 2019.
"And we will be vigilant; we will not accept any backtracking from the UK", Barnier pointed out.
These so-called "Henry VIII" powers also extend to the implementation of the divorce deal agreed with Brussels - something many MPs say is unacceptable.
An array of high-ranking EU officials, including the European Parliament's Brexit coordinator Guy Verhofstadt and European Chief Negotiator for the United Kingdom exiting the EU Michel Barnier, have lashed out at Brexit secretary David Davis' latest remarks on London's concessions during the Brexit talks.
Answering potential Tory rebel Anna Soubry at Prime Minister's Questions, Mrs May warned the amendment could put at risk an "orderly and smooth exit from the EU".
After months of wrangling, May secured a deal last week on three priorities of the separation - Britain's financial settlement, the Irish border and the rights of expatriates.
Ahead of Wednesday's vote, Grieve warned that ministers were asking for "a blank cheque to the government to achieve something that, at the moment, we don't know what it is".
Labour leader Jeremy Corbyn, who will be cheered by the fact that only two of his own Brexiteer MPs rebelled to back the Government, said: "This defeat is a humiliating loss of authority for the Government on the eve of the European Council meeting".