Jeremy Corbyn’s three most senior colleagues broke with their leader to declare Labour a Remain party – earning them cheers at the giant Westminster rally for a Final Say referendum on Brexit.

John McDonnell, the shadow chancellor, Diane Abbott, the shadow home secretary, and Emily Thornberry, the shadow foreign secretary, all made clear Labour’s determination to keep the UK in the European Union.

The speeches prompted chants of “Where’s Jeremy Corbyn?” – to the tune of The White Stripes’ Seven Nation Army, the very chant that symbolised his shock resurgence at the 2017 general election.

The Labour leader, who has suggested he will sit on the fence if the referendum is held, chose not to walk the few yards from the Houses of Parliament to address the rally.

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Meanwhile, former Conservative Antoinette Sandbach, whom Boris Johnson would have hoped to woo to back his deal in next week’s vote, declared her continued support for a fresh public vote.

“When the time comes to support a People’s Vote amendment, I can walk through that lobby with my head held high,” said the MP, currently sitting as an independent.

The speeches were put on hold for the dramatic moment when the result of the Letwin amendment was declared – frustrating Boris Johnson’s hopes of passing his deal – which was greeted with huge cheers.

Labour’s big-hitters arrived soon afterwards, Ms Abbott telling the gathering in parliament square: “I’m a Remainer.”

Ms Thornberry said: “We are internationalists, we are Europeans and we want to stay that way,” adding: “Labour is a Remain party and later we will prove that this is a Remain country.”

Best pictures from Final Say Brexit march Show all 76 1 /76 Best pictures from Final Say Brexit march Best pictures from Final Say Brexit march Crowds march through central London Getty Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march Protesters gather in Parliament Square Led By Donkeys Best pictures from Final Say Brexit march Mayor of London Sadiq Khan joins protesters PA Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march Anti-Brexit protesters fill Parliament Square in London PA Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march A demonstrator marches EPA Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march People's Vote Final say march - 19 October 2019 Angela Christofilou Angela Christofilou/The Independent Best pictures from Final Say Brexit march Protesters push a float depicting Dominic Cummings using Boris Johnson as a puppet during the Final Say Brexit march in London PA Best pictures from Final Say Brexit march EU supporters react after the result of the vote on the deal delay was announced at the House of Commons REUTERS Best pictures from Final Say Brexit march A demonstrator carries his dog draped in EU flag AP Best pictures from Final Say Brexit march Thousands of people taking part in a People's Vote march AFP/Getty Best pictures from Final Say Brexit march Protesters use their flags to shelter from the rain Getty Images Best pictures from Final Say Brexit march Protesters march towards Parliament Square Getty Images Best pictures from Final Say Brexit march Anti-Brexit demonstrators carry placards and EU flags AP Best pictures from Final Say Brexit march Sir Oliver Letwin MP in Parliament Square, London, during an an anti-Brexit, Let Us Be Heard rally, after it was announced that the Letwin amendment, which seeks to avoid a no-deal Brexit on October 31, has been accepted PA Best pictures from Final Say Brexit march Thousands of people taking part in a People's Vote march UK BROADCASTERS POOL/AFP via Get Best pictures from Final Say Brexit march A protester with "Bollocks to Brexit" stickers on his head Getty Images Best pictures from Final Say Brexit march Demonstrators march EPA Best pictures from Final Say Brexit march Britain's main opposition Labour Party shadow Secretary of State for Exiting the EU Keir Starmer, shadow Home Secretary Dianne Abbott, shadow Chancellor of the Exchequer John McDonnell, shadow Foreign Secretary Emily Thornberry speaks on stage in Parliament Square AFP via Getty Images Best pictures from Final Say Brexit march Anti-Brexit supporters cheer outside parliament AP Best pictures from Final Say Brexit march A EU supporter waves flags REUTERS Best pictures from Final Say Brexit march Anti-Brexit supporters AP Best pictures from Final Say Brexit march Protesters shout and chant demanding a final say Getty Images Best pictures from Final Say Brexit march Police look towards protesters Getty Images Best pictures from Final Say Brexit march Pro-Brexit protesters outside the Houses of Parliament in London PA Best pictures from Final Say Brexit march People taking part in an Anti-Brexit, Let Us Be Heard march head to Parliament Square in London PA Best pictures from Final Say Brexit march People taking part in an Anti-Brexit, Let Us Be Heard march in Trafalgar Square PA Best pictures from Final Say Brexit march Anti-Brexit supporters cheer outside parliament AP Best pictures from Final Say Brexit march People taking part in an Anti-Brexit, Let Us Be Heard march past Trafalgar Square PA Best pictures from Final Say Brexit march Protesters shout and chant Getty Images Best pictures from Final Say Brexit march Cory (7) sits on his father's shoulders as protesters march towards Parliament Square Getty Best pictures from Final Say Brexit march People attend the 'Together for the Final Say' march EPA Best pictures from Final Say Brexit march Protesters march towards Parliament Square Getty Images Best pictures from Final Say Brexit march Mayor of London Sadiq Khan speaks on stage PA Best pictures from Final Say Brexit march A woman in EU costume EPA Best pictures from Final Say Brexit march Anti-Brexit protesters fill Parliament Square in London PA Best pictures from Final Say Brexit march Anti-Brexit protesters AP Best pictures from Final Say Brexit march Anti-Brexit protesters hold an Independent banner Angela Christofilou/The Independent Best pictures from Final Say Brexit march Protesters hold an anti-Brexit placard by The Independent stall during the Final Say Brexit march in London Tom Richell Best pictures from Final Say Brexit march Anti-Brexit protesters in Parliament Square, London, PA Best pictures from Final Say Brexit march Demonstrators hold placards and EU flags AFP/Getty Best pictures from Final Say Brexit march An anti-Brexit protester joins in the Final Say Brexit march on 19 October AP Best pictures from Final Say Brexit march EU supporters march in London REUTERS Best pictures from Final Say Brexit march Sir Patrick Stewart and Paul McGann join in the Final Say Brexit march in London on 19 October PA Best pictures from Final Say Brexit march EU supporters march Reuters Best pictures from Final Say Brexit march A demonstrator shouts into a megaphone as she marches AFP via Getty Images Best pictures from Final Say Brexit march A dog dressed in an anti-Brexit placard is walked in the Final Say Brexit march in London EPA Best pictures from Final Say Brexit march EU supporters march REUTERS Best pictures from Final Say Brexit march A morris dancer takes part in the Final Say Brexit march in London PA Best pictures from Final Say Brexit march Whippets are dressed in EU flag clothing during the Final Say Brexit march in London AP Best pictures from Final Say Brexit march Protesters march towards parliament in the Final Say Brexit march in London on 19 October 2019 PA Best pictures from Final Say Brexit march An anti-Brexit demonstrator takes part in the Final Say Brexit march in London AFP/Getty Best pictures from Final Say Brexit march EU supporters call on the government to give Britons a vote on the final Brexit deal Reuters Best pictures from Final Say Brexit march Anti-Brexit protesters Angela Christofilou/The Independent Best pictures from Final Say Brexit march An anti-Brexit protester Angela Christofilou/The Independent Best pictures from Final Say Brexit march Anti-Brexit protesters demonstrate in London during the Final Say Brexit march on 19 October AP Best pictures from Final Say Brexit march Protesters push a float depicting Dominic Cummings using Boris Johnson as a puppet during the Final Say Brexit march in London AFP/Getty Best pictures from Final Say Brexit march Protesters march towards parliament from Park Lane in the Final Say Brexit march in London on 19 October 2019 PA Best pictures from Final Say Brexit march Marchers visit The Independent stall during the Final Say March in London Tom Richell Best pictures from Final Say Brexit march An anti-Brexit protester holds a sign calling for a Final Say during a march in London EPA

And Mr McDonnell, Mr Corbyn’s closest ally, told the crowd that Labour was now committed to staying in the EU, saying: “We believe that our future best lies within the European Union itself.”

Keir Starmer, Labour’s Brexit spokesperson, said the only way to break the impasse was a referendum on the latest deal, adding: “When we get to that vote, we need to fight for Remain.”

A year ago, it would have been unthinkable for Labour’s top team to declare their support for EU membership at a Final Say rally, reflecting the huge shift in the party’s thinking.

They now believe the only way for Mr Corbyn to win an election is by settling the Brexit issue first, at a referendum when Labour will fight for Remain.

Earlier, Star Trek actor Patrick Stewart praised the crowd for proving a second referendum was not the “pipe dream” or “moon shot” that it was seen as when the campaign began.

He remembered how it had begun for him in “an old night club in Camden”, in North London, saying: “You haven't just filled a nice bar in north London, you have taken over an entire city.

“You haven't just impacted the Brexit debate, you have transformed British politics.”

And Great British Bake Off presenter Sandi Toksvig likened the Brexit deal on offer to a rickety old car, with the door missing entirely on the passenger’s side.