Labour has been plunged into a new bout of infighting over Brexit after 50 MPs defied the leadership to back a Commons motion calling for Britain to remain in the single market.
Supporters of the Jeremy Corbyn rounded on the rebels accusing them of indulging in the "worst kind of gesture politics", undermining party unity.
The Labour leader sacked three shadow ministers who joined the revolt, while a fourth announced he was quitting the front bench.
However the rebels' ringleader, former shadow business secretary Chuka Umunna, vowed to carry on working with pro-European MPs from across Parliament to prevent a "hard Brexit".
The row erupted as the Commons voted on Thursday to approve the Queen's Speech with the Democratic Unionists (DUP) supporting the Conservatives, giving Theresa May a majority of 14.
But it passed only after ministers rushed out an announcement of extra funding for women in Northern Ireland to have abortions on the NHS in England, to head off a threatened revolt by Tory MPs.
There was frustration within the Labour leadership that focus was once again on the party's divisions on Europe, just as it appeared to be coming together after its unexpectedly strong showing in the general election.
Forty-nine Labour MPs voted for Mr Umunna's amendment calling for the UK to continue membership of the European single market and the customs union, while a 50th acted as one of the tellers.
Union boss Dave Prentis said it was "totally inappropriate" for MPs to "create" a split over Europe just as the Government was coming under pressure over public sector pay.
"Parliament was just seven votes shy of overturning the Government's cruel pay cap, yet instead of keeping up the pressure, some seem determined to let the Conservatives off the hook," the Unison general secretary said.
"Undoubtedly Europe is and will remain an important issue, but when unity is so important in the Labour Party, it is utterly self-defeating to become bogged down in the worst kind of gesture politics."
Labour deputy leader Tom Watson said the vote had been "unnecessary" and that he was "very disappointed" with Mr Umunna for moving it
"To break away like that is politically unhelpful at a time when the entire Labour Party is buzzing because we did far better in the general election," he told Channel 4 News.
"I just hope we can come back together very quickly."
In the event the amendment was defeated by 322 votes to 101 with the rest of the Labour Party abstaining.
But with the SNP and Liberal Democrats backing the amendment, Mr Umunna said the fight against a "hard Brexit" would continue.
"What it shows is that pro-European backbench MPs of all parties are not going to simply submit to a hard Brexit," he said.
"With a hung parliament, we have a real chance in future to shape Britain's exit from the EU to protect jobs and working people."
Lib Dem leader Tim Farron denounced Mr Corbyn for failing to back the amendment, saying voters would be left feeling betrayed.
"Millions of people who voted for Jeremy Corbyn were hoping for a new approach to Brexit," he said.
"They will be feeling utterly betrayed that he has yet again failed to oppose this government's extreme Brexit agenda."