Jeremy Corbyn suffered a significant rebellion against his order to vote for Brexit on Wednesday evening, as a quarter of Labour MPs voted against triggering Article 50.
The European Union (Notification of Withdrawal) Bill, which will allow Theresa May to start the Brexit process, cleared its first parliamentary hurdle by a vote in the Commons of 498 to 114.
The Labour leader saw 47 of his MPs rebel against him and vote against triggering Article 50 - compared to 167 who backed his position.
Ahead of the vote, two more of Corbyn's frontbench team quit in order to allow them to defy his three-line whip.
Shadow environment secretary Rachael Maskell and shadow equalities minister Dawn Butler both resigned. Shadow Welsh secretary Jo Stevens had already quit, as had shadow minister for early years Tulip Siddiq.
SNP and seven of the nine Lib Dem MPs also voted against triggering Article 50.
This evening's vote came after two days of lengthy debate in the Commons. The government was forced to seek parliament's approval for its plans by a Supreme Court ruling last week.
Long riven by splits over Europe, both pro-Remain and pro-Brexit Conservative MPs, with the exception of Ken Clarke, have united behind triggering Article 50.
But the EU referendum has caused a split in the Labour Party. Corbyn has struggled to convince many of his MPs to obey his orders to back the Bill.
The overwhelming majority of Labour MPs backed Remain - but most Labour constituencies voted to Leave.
The Labour leadership has argued it would be undemocratic for the MPs to ignore the national referendum result.
Many were not convinced. Siddiq told the Commons earlier today she would be "abandoning" her Remain voting constituency if she backed Article 50.
Former shadow cabinet minister Chris Bryant said he would be voting against Article 50 even though the majority of his constituents backed Leave.
Lewisham West MP Jim Dowd said he could "not be complicit in something I know and feel is wrong".
And Neil Coyle, the MP for Bermondsey and Old Southwark, was told off by the Speaker for branding the Conservatives "a whole government full of bastards" as he explained why he would reject the Bill.
Ken Clarke, the only Tory MP who voted against the Bill, yesterday hit out at the Alice in "Wonderland" expectations of Brexiteers.
"No sensible country has referendums," he said. "I admire my colleagues who can suddenly become enthusiastic Brexiteers, having see a light on the road to Damascus. That light has been denied me."
But the rest of the pro-EU Tories fell into line and have reconciled themselves to trying to soften the impact of Brexit.
George Osborne, one of the main architects of the Remain campaign and who saw his frontbench career destroyed by the result, said if MPs blocked Brexit it would "provoke a deep constitutional crisis" as the referendum result had to be obeyed.
And the mild-mannered pro-EU Tory Alistair Burt said while the referendum had "soured friendships, deepened bitterness and damaged relationships" to the extent that he "swore at a mate in the tea room" - he would be voting for Article 50.
The real parliamentary battle is not expected to take place until next week, when opposition parties attempt to push through a series of amendments as the Bill undergoes detailed scrutiny in its committee stage. It must also navigate its way through the House of Lords.