Jeremy Corbyn has declared Labour will back a second referendum on Brexit to end a lengthy power struggle within the party.
The Labour leader formally threw his support behind the campaign for a national poll after his own alternative plan for a Brexit deal was rejected by MPs.
“We will back a public vote in order to prevent a damaging Tory Brexit or a disastrous no deal outcome,” he said.
Mr Corbyn declared the party would also “continue to push for the other available options” to prevent a no-deal divorce with the European Union.
Shadow chancellor John McDonnell later confirmed Labour will attempt to secure a second referendum as soon as Prime Minister Theresa May brings her Brexit deal back to the House of Commons, expected to be on 12 March.
“That’s the time when we will have to put the amendment up,” he told ITV’s Peston show.
Raising the prospect of a confirmatory referendum on a Brexit deal, Mr McDonnell said: “Either a deal will go through which will protect jobs and the economy or, to get some deal through, it will be conditional on going back to the people.”
But, he also stressed Labour “are still going to argue that we want a general election, we are still going to argue we think our deal that we have put up was the best option”.
The announcement of Labour’s support for a fresh public vote marks a significant shift in the party’s strategy.
Mr Corbyn poured cold water on the campaign for a “People’s Vote” in an exclusive interview with Sky News last November.
He called it an “option for the future… not an option for today”, after having warned the week before to German media that “we can’t stop” Brexit.
Then-Labour MPs admitted there would never be a referendum on Brexit without Mr Corbyn’s support.
Luciana Berger, who has since quit Labour to join the new Independent Group of MPs, accused him of “facilitating a jobs-destroying Brexit”.
A row over Labour’s Brexit policy was decided at its national conference last September.
An agreed motion vowed to push for a general election, but added if the party could not get one then “Labour must support all options remaining on the table, including campaigning for a public vote”.
After tabling unsuccessful no-confidence votes in Mrs May and her government last month, Mr Corbyn’s own plan for a “close economic partnership” – including a permanent customs union – with the EU was also defeated on Wednesday night.
Pro-EU campaigners have welcomed the news of Labour’s support for a new public vote, although Mr Corbyn did not say what options would be on the ballot paper in such a referendum.
A Labour source heavily hinted that a Remain option would be on the ballot paper.
Mrs May has previously ruled out another referendum on Brexit, with her party chairman Brandon Lewis saying it would “take our country back to square one”.
Despite Mr Corbyn’s backing, there are still several Labour MPs who are deeply opposed to a fresh national poll.
Shadow justice secretary Richard Burgon suggested there was more time for the party to push for its own version of Brexit before being forced to back another referendum.
“Given parliament looks set to extend Article 50, there’ll be further opportunities to secure support for Labour’s alternative Brexit deal,” he said.
Article 50 is the legal mechanism in EU treaty law that takes the UK out of the bloc.
Labour MP Julie Cooper wrote she had “no intention of voting for a second referendum”.
Wednesday night’s dramatic Commons showdown also saw Mrs May formally accept two amendments.
Tory MP Alberto Costa was forced to resign for submitting his plan to protect EU and UK citizens’ rights in the event of a no-deal divorce, despite the government agreeing to his proposal.
And Labour’s Yvette Cooper bid to formally confirm Mrs May’s commitment to give MPs further votes on endorsing no-deal or delaying Brexit – if there is still no agreement ratified by parliament by mid-March – was also accepted by the government.
The UK is on course to leave the EU on 29 March under current legislation – with or without a deal.