Nigel Farage’s Brexit Party are set to win next month’s European Parliament elections, according to a new poll.
The former UKIP leader’s party is on 27%, ahead of Labour on 22% and the Conservatives on 15%.
The poll – commissioned by the People’s Vote campaign for a second referendum and weighted by likelihood to vote – follows the Brexit Party’s official launch last week.
Its findings will heighten fears in Downing Street that the Tories are on course for a damaging defeat that will likely reignite calls for Theresa May to resign.
Britain looks set to take part in the 23 May poll amid the continuing Brexit deadlock at Westminster.
The prime minister has seen her deal rejected by MPs three times, prompting her to seek two delays to Brexit.
Britain is now due to leave the EU on 31 October, seven months later than planned.
Mrs May has been holding cross-party talks with Labour and Jeremy Corbyn to try and find a way out of the impasse.
The government remains hopeful of getting a deal passed in time to avoid taking part in the EU elections, but contingency planning is under way in case they need to go ahead.
If they do, the results will be closely watched as a barometer of public sentiment on Brexit.
The YouGov poll suggests that part of the increase in support for the Brexit Party is down to UKIP voters switching to the party – with UKIP down from 14% in a poll last week to 7% now.
Among the other parties, the Greens are on 10%, the Liberal Democrats on 9%, newly-formed Change UK on 6% and the SNP/Plaid Cymru on 4%.
The People’s Vote campaign said the poll suggested Labour could potentially stop the Brexit Party finishing first if it came out in favour of a public vote.
In those circumstances, the poll says support for Labour would increase slightly to 23%, while support for the Brexit Party would drop to 26%.
However, if the party’s manifesto commits it to proceeding with Brexit its support drops to 15%, level with the Conservatives and the Lib Dems.
Dame Margaret Beckett, a Labour former foreign secretary and People’s Vote supporter, said if the party “hedge our bets” on another public vote, 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.”
Lib Dem leader Sir Vince Cable said Remain-supporting political parties should be “fighting together under the same banner” in the elections.
Sir Vince acknowledged there was “not a great deal” of difference between his party’s message and that of Change UK, which is made up of 11 MPs who have left Labour and the Tories.
He told BBC Radio 4’s Today programme: “So, there’s a variety of different parties offering the same message, something which is possible under the proportional voting system that we have.”
He added: “No, it’s not crazy, I mean 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, and 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.”
:: YouGov questioned 1,855 British adults between 15 and 16 April.