Polls make Vince Cable’s party the favourite for remainers and put it in first place in London

Senior Labour figures were engaged in a desperate battle to shore up the party’s support on Saturday night, amid warnings that its stance on Brexit was helping to “detoxify the Lib Dems”.

With just days left before the European elections at which Nigel Farage’s Brexit party is expected to triumph, shadow cabinet ministers are among those concerned that Labour’s ambiguous position on Brexit has helped revive the Lib Dems. It comes as new polling seen by the Observer suggests Vince Cable’s party is running in first place in London and could even beat Labour overall.

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One senior party figure warned: “If the consequence of Labour’s Brexit position and this European election is to essentially detoxify the Lib Dems, then that’s a real problem.” Clive Lewis, a shadow Treasury minister, said “lifelong Labour voters” would not back the party this week due to its Brexit stance. He added: “It feels like we’ve given [the Lib Dems] the political equivalent of resuscitation.”

The fears came as it emerged that:

• London mayor Sadiq Khan has pledged that he will campaign for Remain “day and night” in any second referendum, as he urged voters, in an article for the Observer, to back Labour to stop a Farage victory.

• Theresa May is planning to make one last “bold offer” to MPs in a desperate attempt to secure support for her Brexit deal, including changes demanded by Labour, Democratic Unionist and Tory MPs and a new round of indicative votes.

• Labour MPs revealed they were already drawing up attempts to stop a no-deal Brexit should a hardline Brexiter replace May as prime minister, with some suggesting that revoking Brexit had to be a fall-back option.

An Opinium poll for the Observer found that the Liberal Democrats have narrowly overtaken Labour as the favourite party of remain voters, with 29% of the group now backing Cable’s party and 28% backing Labour.

Meanwhile, a wider poll found that the Lib Dems are set to top the poll in London and could even push Labour into third place overall in the European election.

The YouGov/Datapraxis poll of more than 9,000 voters, commissioned by the pro-remain Best for Britain campaign and Hope Not Hate, found the Brexit party leading in all other regions apart from Scotland, where the SNP leads.

It found that the Lib Dems had overtaken Labour, once the pollster had removed voters who said they did not know how they would vote, or would not vote. It put the Lib Dems on 17%, Labour on 15% and the Greens just four points further back, on 11%. The Brexit party was leading on 34% and the Tories were on 9%.

A Tory cabinet minister also told the Observer they were worried about the Lib Dems, as remain supporters abandoned their party. “I’ve been warning for some time that the Lib Dems are not dead and could come back, but have been ignored,” they said. “If you map the local elections on to a general election, there are seats in which big majorities of over 10,000 votes could be wiped out.”

Serious concerns are being expressed among Labour MPs. One senior frontbencher said: “Labour should take the Lib Dem revival seriously.

“If the consequence of Labour’s Brexit position and this European election is to essentially detoxify the Lib Dems, then that’s a real problem for a party that is coming from younger voters, students, people who rent – a whole cohort of voters who in previous times may have voted Lib Dem. We can’t afford to lose them.”

Facebook Twitter Pinterest Brexit party leader Nigel Farage campaigning in South Ockendon. Photograph: Joe Giddens/PA

Lewis said: “The Liberal Democrats were flatlining, rightly tainted with their association with austerity and the Tory government. Ultimately, the position we’ve taken on a confirmatory vote means that lifelong Labour voters are seeing the European elections as a kind of EU referendum, part two – to send a message to the two main parties.

“Many people are telling me that they are not going to vote Labour for the first time in their lives. You’re breaking a seal. The first time is the most difficult, but the second and third time could become easier.”

Neil Coyle, a Labour MP in a former Lib Dem seat, said: “Jeremy Corbyn has breathed life back into the Lib Dem carcass by appearing to be bailing out May. It’s an astonishingly damaging position and members are very angry. By appearing to face both ways on Brexit, voters are slapping both Labour’s faces.”

However, leader Jeremy Corbyn’s team is adamant the party has to try to retain Labour’s leave and remain voters. Labour called off cross-party talks last week, but has not ruled out backing a process of votes to find a way out of the Brexit impasse. At a rally on Saturday, Corbyn called on voters to back Labour to halt the “far right”.

Writing in the Observer, Khan pleads with progressive voters to back Labour to keep out Farage. With the Lib Dems surging in London, he also commits to campaigning “day and night for Britain to stay a member of the European Union” in a future second referendum.

“We must stand united in our opposition to the rise of the far right and reject their dystopian vision of our future. Labour is the only party that can beat them,” he writes. “Voting for smaller parties, meanwhile, just makes Nigel Farage’s job easier – and makes it more likely that we wake up to headlines proclaiming him as the winner.”

The latest polling will alarm Labour insiders. Naomi Smith, from Best for Britain, said: “Our poll is the largest yet, and shows that there’s still everything to play for in these European elections. It’s fundamentally important that as many people as possible get out and vote, especially those who often get forgotten by politicians – young people, renters and minority voices. No matter their frustrations with politics, staying at home on polling day is never the answer.”

Nick Lowles, from Hope Not Hate, said: “These results are incredibly alarming. They envision a populist right party storming to victory in these elections. If that happens as this poll suggests, it will be a big boost for the forces of division in this country.”