Labour’s annual conference has voted to reject a bid to commit the party to campaigning for Remain in any future EU referendum.

The result bolsters the position of leader Jeremy Corbyn, whose proposal to promise a referendum but wait until after the general election to decide which side to back was passed overwhelmingly.

And it marks a formidable setback to anti-Brexit campaigners who had hoped to secure Labour’s support in the fight for Remain.

It means Labour will almost certainly go into the general election expected within the next three months without a clear position on whether it would campaign to stay in the EU or leave in a referendum.

Shadow cabinet minister Diane Abbott hailed the vote – which was greeted by choruses of “Oh Jeremy Corbyn” from the conference floor – as a display of unity behind the leader. But Scottish first minister Nicola Sturgeon denounced it as an “abdication of responsibility”, while Conservative chair James Cleverly said it showed Labour was offering only “more talk, more indecision and more pointless delay”.

There was anger after the crucial vote, which took place amid chaotic scenes at the Brighton gathering, as disputes broke out over whether the vote by a show of hands had been passed or not.

National Executive Committee official Wendy Nichols, who was chairing the session, admitted she had initially thought the vote was in favour of Remain, but was told by general secretary Jennie Formby that delegates had in fact voted against. She resisted angry demands for a card vote to get a precise measure of the balance of opinion.

Under rule changes agreed last year and implemented at this conference, delegates no longer have the right to demand a card vote, which is instead at the discretion of the chair.

Brexit and climate protests at Labour party conference: In pictures Show all 12 1 /12 Brexit and climate protests at Labour party conference: In pictures Brexit and climate protests at Labour party conference: In pictures Labour leader Jeremy Corbyn walks past anti-Brexit campaigner on the second day of the party conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures A protester holds a sign mocking Labour leader Jeremy Corbyn's Brexit stance at the party conference in Brighton EPA Brexit and climate protests at Labour party conference: In pictures Activists from anti-climate change movement Extinction Rebellion march on the Labour conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures Delegates take part in an informal vote on Labour's Brexitr stance at the party conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures Shadow foreign secretary Emily Thornberry leads a march in support of a second referendum on Brexit Getty Brexit and climate protests at Labour party conference: In pictures Anti-Brexit protesters take part in a People's Vote rally during the Labour party conference in Brighton EPA Brexit and climate protests at Labour party conference: In pictures Extinction RebelliActivists from anti-climate change movement Extinction Rebellion march on the Labour conference in Brightonon EPA Brexit and climate protests at Labour party conference: In pictures Extinction Rebellion activists march on the Labour conference in Brighton AFP/Getty Brexit and climate protests at Labour party conference: In pictures A delegate arrives carrying a briefing from the Labour Representation Committee to the Labour party conference in Brighton Getty Brexit and climate protests at Labour party conference: In pictures Prominent anti-Brexit campaigner Steve Bray protests on Brighton beach during the Labour party conference Reuters Brexit and climate protests at Labour party conference: In pictures Anti-Brexit protesters take part in a People's Vote rally during the Labour party conference in Brighton EPA Brexit and climate protests at Labour party conference: In pictures Activists raise a banner calling for the Labour party to restore the original Clause 4 of its constitution, which would pledge the party to nationalising all industry Getty

One Labour Remainer said: “It was the grassroots against the party machine and the machine won.”

Remain-backers’ hopes were boosted earlier in the day when a string of trade unions, led by the country’s largest union Unison, came out in favour of a motion tabled by more than 80 constituency Labour parties, calling for the party to campaign “energetically” for continued EU membership in any referendum.

Shadow Brexit secretary Sir Keir Starmer left little doubt where his sympathies lay, telling delegates moments before the vote: “I have a very simple message today: if you want a referendum – Vote Labour. If you want a Final Say on Brexit – Vote Labour. If you want to fight for Remain – Vote Labour. Labour will let the people decide”.

Starmer was joined by fellow frontbenchers Emily Thornberry and Clive Lewis at a large People’s Vote rally on the first day of the conference, at which many delegates demanded outright support for Remain.

Speaking to Politico shortly after the vote, Starmer said it was still “likely” that the party would eventually fight for continued EU membership in a referendum.

“Obviously I am disappointed by the result,” he said. “But we had a vote. That’s what the Labour Party does.”

Sir Keir said he would continue personally to campaign for Remain, and said that the NEC position represented “quite considerable movement” over the past 12 months.

Emily Thornberry and Sir Keir Starmer march at the anti-brexit Trust the People rally (PA)

Mr Corbyn’s plan, approved by a show of hands, envisages a Labour government negotiating a “credible” Brexit deal within three months of taking power, with the UK remaining in the EU’s customs union and maintaining a close single market relationship, while safeguarding European-style workplace, environmental and consumer protections.

The plan commits Labour to a referendum within six months, but leaves the question of which side the party would take to be decided at a special conference after the election. Starmer announced that Labour would legislate to fix the date of the referendum ”immediately” after taking office.

Following the vote, close Corbyn ally Ms Abbott said: “The message is – despite the chatter from commentators – the party is determined to unite behind its leader.”

But Michael Chessum of the pro-EU Another Europe Is Possible group, which drafted the Remain motion, said: ”Labour members, 90 per cent of whom want to stay in the EU, will be deeply disappointed with this decision. It is possible that the Remain motion had a majority in the constituency Labour parties, but because there was no card vote we will never know.”

Mr Chessum said it was “not too late for Labour to make clear that it opposes Brexit outright”.

“Though it is not the policy we supported, the calling of a special conference to democratically decide Labour’s Brexit policy was a concession which we won,” he said. “But a fudge is not a unity position. It is deeply divisive among members, and risks losing a large chunk of our voter base.”

Andrew Lewin, founder of Remain Labour, said “We were told the Labour Party would embrace democracy, but today it was the grassroots against the party machine – and the machine won.

“If this fudge is the Labour policy at the next general election, we will drive Remain voters away. It simply isn’t viable to be ‘neutral’ on Brexit.”

Liberal Democrat leader Jo Swinson, whose own party committed to revoking Brexit at its conference last week, said: “Jeremy Corbyn has again shown a total lack of leadership on Brexit and settled on yet another fudge on the biggest issue facing our country. Jeremy Corbyn has repeatedly had the opportunity to put the full force of the Labour behind a Remain position, but he has once again shown today that he is a Brexiteer at heart.”

Ms Sturgeon tweeted: “Labour rejects a clear pro-Remain position and instead stays firmly perched on the Brexit fence... It beggars belief – from a practical and principled point of view – that Corbyn thinks he can go through a General Election without saying if he backs Remain or Leave. The SNP’s position is unequivocally Stop Brexit.”

Labour’s mayor of London Sadiq Khan said: “I do not believe this decision reflects the views of the overwhelming majority of Labour members who desperately want to stop Brexit. Labour is a Remain party. I will continue campaigning with London Labour to give the public the Final Say and stop Brexit.”

Labour MP Ian Murray, a prominent campaigner for Remain, said: “It’s important to recognise that we now have a policy of a public vote on any deal with the option to Remain. That’s really the only show in town.

“But it is of course deeply disappointing that we have not chosen to campaign for Remain at this stage, as that certainly does not reflect the strong views of the overwhelming majority of our members and supporters.

“It is simply not tenable for our leadership to be neutral when we face the biggest crisis our country has witnessed in modern times.”

And Green MP Caroline Lucas said: “After the climate crisis, the biggest issue we face – and Labour refuses to take a side. At a time when we need a strong opposition more than ever, they’ve left the pitch. This is a tragedy for our politics and our country.”