Worried backbenchers piled pressure on the Labour leader to shift his stance before the 23 May poll, after the former Ukip leader was revealed to be on course to triumph at the head of his new Brexit Party.
A survey gave Mr Farage’s party a healthy five-point lead at 27 per cent of the vote, leaving both Labour (22 per cent) and the Conservatives (15 per cent) trailing in his wake.
Labour supporters of a Final Say referendum seized on evidence that Mr Corbyn was heading for a further disastrous slump if his manifesto backed forcing through a Brexit deal in alliance with Theresa May.
A customs union deal – the aim of the current cross-party talks – would see Labour dip to just 15 per cent, according to the poll of 1,855 adults by YouGov, handing the Brexit Party a 10-point win.
But undiluted support for a further public vote would lift Labour to 23 per cent, with Mr Farage’s outfit only three points ahead.
Owen Smith, a former Labour leadership contender, told The Independent: “It’s very clear that Labour is losing support among our voters because the leadership has refused to give unambiguous support to a people’s vote on Brexit.
“We should never forget that the majority of Labour supporters voted Remain in 2016 and if we want to beat the Brexit parties we have to honour their views.”
Stephen Doughty, a former shadow minister, echoed the fear, saying: “We must put a public vote on Brexit at the core of our European manifesto
“Pro-European voters who in all other respects support Labour need to see that message loud and clear. Otherwise we risk leeching support to other parties – which can only benefit Farage and his forces.”
Sir Vince Cable, the Liberal Democrat leader, also said it “would be a game-changer” if Labour came out clearly to campaign to stay in the EU.
But he admitted: “I find it difficult to see they could do that given that Jeremy Corbyn has said repeatedly he is there to deliver Brexit, but it certainly would change the nature of the argument.”
Mr Farage’s surge follows the burst of publicity the Brexit Party received at its campaign launch last week, when Annunziata Rees-Mogg – the sister of the leading Tory Brexiteer Jacob Rees-Mogg – was unveiled as a candidate.
It also heightened Tory fears that their party is heading for a crushing defeat, which would trigger fresh calls for the prime minister to quit.
Justine Greening, the former Conservative education secretary, hinted she would quit if that resulted in her party adopting a harder Brexit position.
“It’s certainly a challenging time I think for me to be in the Conservative Party,” she said. “For me it was about three things: opportunity, a strong economy and well-managed public finances.
“And clearly I think if we become the Brexit party, that really goes against those three core tenets of what I think being a Conservative Party member is all about.”
He revealed his party had proposed fighting together – a move that one election expert has predicted could deliver an extra six seats in Brussels.
Frustrated campaigners for a Final Say public vote also believe a unified campaign would have excited voters and delivered an even greater reward.
“It would be better, I think, from the point of view of the supporters of British membership of the EU if we were fighting together under the same banner,” Sir Vince said.
“Certainly that’s something we would like to have seen, but that wasn’t possible, we didn’t get a positive reaction to that, so we are going on our own.”
Change UK, the new party name for the Independent Group, has said it wants “no alliance and no pacts, but to be a new party standing on its own”, a stance echoed by the Green Party.
The YouGov poll put the Greens top among the pro-Remain parties on 10 per cent, ahead of the Liberal Democrats (9 per cent) and Change UK (6 per cent).
The Independent revealed today that experts believe the prime minister has already run out of time to stop UK participation in the elections, because her deal cannot be fully ratified by 22 May.