Marine Le Pen's National Rally (RN) could block Britain's return to the EU if it wins power at the next election, senior party officials have indicated.
Labour leadership contenders have weighed up a potential to return to Brussels after the next General Election - but such a move could only be done with the consent of all of the union's 27 member states.
The RN's president, Jordan Bardella, is widely tipped as the favourite to clinch victory in next year's French presidential elections - and could veto the reversal, according to his economic adviser Charles-Henri Gallois.
Mr Gallois - who also represents the party as an MEP - said RN may veto any move to return Britain to the EU unless the Government held a second referendum.
He told The Telegraph: “To do so without a referendum would obviously be a denial of democracy because the people expressed their will through a referendum to leave.
“It’s a bit of a special case, but yes we would oppose it without a referendum, I think, because in any case we’re against any enlargement of the EU anyway.
“It seems perfectly normal to me, given that there was a referendum to leave, that we can’t accept a country rejoining without a referendum.”
The RN official's intervention comes as the debate surrounding Britain's membership of the EU was reignited by several would-be Labour prime ministers.
Ms Le Pen's party are widely tipped to storm to victory in next year's French presidential elections
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Launching his leadership bid over the weekend, Mr Streeting declared that Britain's future "lies with Europe, and one day back in the European Union", branding Brexit "a catastrophic mistake".
Andy Burnham - who echoed similar sentiments at last year's Labour conference - appeared to walk back on his previous stance, saying Brexit had been "damaging" but "the last thing we should do right now is re-run those arguments".
Meanwhile, Sir Keir Starmer has refused to rule out a return to the political union one day, insisting he would "not get lost in a debate about what might happen years down the line".
Jean-Philippe Tanguy, RN's deputy parliamentary leader, lambasted the prospect of a Brexit reversal without another referendum, declaring: “Quelle horreur!”
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Mr Gallois said RN may veto any move to return Britain to the EU unless Labour held a second referendum
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But if Reform UK wins power at the next election, the RN may not need to intervene.
Mr Gallois insisted he did not believe his party would have to act to stop a "Breturn" - even if it lost next year's election.
He said: “Nigel Farage will win the General Election before there’s any talk of a referendum or anything like that. And so, that will put an end to that possibility once and for all."
However, the RN MEP warned that if pro-EU centrists clinched victory over his party, they would seek to "punish the UK out of a sort of quasi-religious fanaticism".
Ms Le Pen has long-been an ardent backer of Brexit, hailing the British electorate's decision to walk away from "the EU poison" as the “most important moment since the fall of the Berlin Wall”.
Mr Streeting declared that Britain's future 'lies with Europe, and one day back in the European Union'
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Mr Bardella previously held talks with Nigel Farage in London, seeking to forge an alliance with Reform UK
| X/J_BARDELLAShe campaigned on leading France out of the EU via a referendum in her bid to become French President in 2017, but was subsequently defeated by Emmanuel Macron.
However, Ms Le Pen will not be allowed to stand as RN's candidate next year after being found to have embezzled EU funds - leaving Mr Bardella to lead the charge.
In December, he held talks with Nigel Farage in London, seeking to forge an alliance with Reform UK to "restore Europe's borders".
The RN candidate also vowed that Mr Farage “will be the next Prime Minister”, describing the meeting as preparations for government.
Mr Bardella was full of praise for his British counterpart after the talks, labelling Mr Farage a "pioneer" who fought for Britain's independence from the EU.
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