Jeremy Corbyn has suffered a major rebellion in the Labour ranks after more than a third of his MPs voted against the whip over the Brexit Bill.
A total of 89 of the party's 257 MPs ignored front bench orders to abstain from voting either for or against an amendment to the EU (Withdrawal Bill) that sought to prioritise European Economic Area (EEA) membership in EU negotiations.
They included six Labour MPs who quit senior roles minutes before the vote - which saw the amendment defeated by 201 - in order to rebel against the party's abstentionist position.
But the rebels were split, with 74 voting in favour of the Lords amendment, which called for the Government to pursue the so-called "Norway Model" membership, and 15 against it.
Laura Smith stepped down as a junior shadow cabinet office minister to vote with the Government, saying "remaining in the EEA is not the right way forward for our country".
But five parliamentary private secretaries (PPS) voted in favour of the EEA amendment; Ged Killen, Ellie Reeves, Tonia Antoniazzi, Anna McMorrin and Rosie Duffield, with all but Ms Duffield announcing their resignation before the vote.
Mr Corbyn said: "I understand the difficulties MPs representing constituencies which voted strongly for Leave or Remain have on the EEA amendment to the EU Withdrawal Bill.
"The Labour Party respects the outcome of the EU referendum and does not support the EEA or Norway model as it is not the right for option for Britain."
An amendment introduced by Labour, which called for access to the single market but stopped short of EEA membership, was defeated by 82 votes, with no party rebels.
The Conservatives said the resignations showed that Mr Corbyn "can't lead his own party let alone our country through complex Brexit negotiations".
Lib Dem leader Sir Vince Cable accused Labour of wasting an opportunity to defeat the Government, accusing Mr Corbyn of being "completely defunct as an Opposition Leader".
He said: "Brexit is not inevitable, but we do need Labour's front bench to do their job and join us in taking on the Tory government."
Just three Conservative MPs voted in favour of the Lords amendment, arch-Remainers Ken Clarke, Dominic Grieve and Anna Soubry.
After days of division and bitter rows between Brexiteers and Remainers in the Tory ranks, Theresa May was able to see off another amendment that would have tied Britain to the customs union post-Brexit by 325 votes to 298, majority 27.
But the Prime Minister is likely to face the risk of further rebellions further down the line.
At Prime minister's Questions on Wednesday Mrs May said that Parliament cannot be allowed to "overturn the will of the British people" on Brexit.
The Prime Minister was speaking as she confirmed that the Government will table a new amendment to her flagship EU Withdrawal Bill, setting out in more detail the terms of the "meaningful vote" promised to MPs on the final Brexit deal.
Mrs May saw off a threatened defeat on the issue in the Commons on Tuesday by assuring would-be rebels personally that she would take their concerns on board.
But pro-EU Tories are warning that they remain ready to rebel if their demands are not satisfied by the compromise amendment, expected to be tabled on Thursday ahead of the bill's return to the Lords on Monday.
Mrs May was responding to a question from Conservative arch-Brexiteer Jacob Rees-Mogg, who said it was vital that any amendment preserved the separation between the roles of Government and Parliament.
Mr Rees-Mogg later used an op-ed in the Times to accuse meaningful vote-backers of wanting to stop Breit, saying the amendment would "introduce an unworkable constitutional proposition that on its own would make no deal more likely."