Nigel Farage's new Brexit Party has opened up a five-point lead in the next month's elections to the European Parliament, according to a new opinion poll.
A YouGov poll, commissioned by the People's Vote campaign, puts the Brexit Party on 27%, ahead of Labour on 22% with the Conservatives trailing on 15%.
The findings - which are weighted by likelihood to vote - represent a surge in support for the new party since a YouGov poll for The Times last week put them on 15% - almost level-pegging with the Tories on 16%, with Labour leading on 24%.
It follows the burst of publicity the Brexit Party received with the launch last week of its election campaign, when it was announced that Annunziata Rees-Mogg - the sister of the leading Tory Brexiteer Jacob Rees-Mogg - would be among its candidates.
It will reinforce fears among ministers that the Conservatives are heading for a crushing defeat if the poll on May 23 goes ahead as planned - a result which would almost certainly see fresh calls for Theresa May to quit.
The Prime Minister has said she is determined to get a Brexit deal through Parliament before that date, which would mean voting would be cancelled.
However, that not only means winning a "meaningful vote" on a deal - which has already been rejected three times by the Commons - but also then passing a bill formally ratifying the agreement in law.
Much is likely to depend on whether cross-party talks with Labour can agree a common way forward - with the two sides expected to take stock of progress when MPs return to Westminster after the Easter recess.
The latest YouGov poll suggests that some of the increase in support for the Brexit Party comes from Ukip voters switching to the new party, with Ukip down from 14% last week to 7%.
Among the pro-Remain parties, the Greens came top with 10%, followed by the Liberal Democrats on 9%, Change UK on 6% and the SNP/Plaid Cymru on 4%.
Meanwhile, the People's Vote campaign - which backs a second referendum - said the findings suggested Labour could stop the Brexit Party topping the poll if it backed a public vote on whether to go ahead with Brexit.
The poll showed that in those circumstances, support for Labour would increase slightly to 23%, while support for the Brexit Party dropped to 26%.
However if Labour's manifesto commits it to going ahead with Brexit - even with a customs union which it is seeking to negotiate in the talks with the Government - its support drops to 15%, level with the Conservatives and a resurgent Liberal Democrats.
Labour former foreign secretary and People's Vote supporter Dame Margaret Beckett said if the party "hedge our bets" on a referendum, the Brexit Party would storm to victory.
"These elections have proven to be rich hunting grounds for Nigel Farage's brand of extreme right wing politics before and may be again," she said.
"But the message of this poll is loud and clear: it suggests that if anyone can stop Farage winning it is Labour - and only if we back a people's vote."
Liberal Democrat leader Sir Vince Cable said the campaign would be transformed if Labour came out in favour of remaining in the EU in a second referendum, although he doubted Jeremy Corbyn would be willing to do so.
"It would be a game changer if they made it absolutely clear that in a referendum campaign they would campaign to remain within the EU," he told the BBC Radio 4 Today programme.
"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."
:: YouGov questioned 1,855 GB adults between April 15 and 16.