Sadiq Khan, the Labour mayor of London, has called on Jeremy Corbyn to opt for a second referendum before a general election, saying it would be both electorally damaging and fundamentally wrong to continue with the party’s current Brexit policy.

Khan, one of the party’s most powerful office-holders, called on Labour to ballot its members on whether the party should immediately back remaining in the EU, branding the existing position a “fudge” that risked losing the next election.

Speaking at his offices in City Hall, he said it was both “the wrong policy and also electorally bad” for the party to go into an imminent general election without telling voters Labour is in favour of staying in the EU.

“I am not sure, bearing in mind how big Brexit is, that we can have a general election and argue on NHS, policing, schools, the environment with Brexit not being resolved and our position being so unclear; it’s bad politics and the wrong place for us to be,” he said.

In a wide-ranging interview six months before Khan comes up for re-election as mayor, he said his decision to speak out was not related to any worries about his own prospects, arguing that he had always been clearly pro-remain. Instead, he claimed he felt obliged to warn the national party against making a huge mistake.

His intervention comes at a time when Corbyn is under increasing pressure from senior shadow cabinet members to agree to a second referendum before an election, with many of them arguing it is crucial to first get Brexit sorted. The party is trailing in the polls, with the Lib Dems appearing to attract pro-remain voters with their position in favour of revoking Brexit, while Labour continues to refuse to say which way it would campaign in a second referendum.

The mayor launched a fresh broadside at Labour’s Brexit policy just three weeks after the party’s autumn conference, at which delegates backed Corbyn’s policy of fighting an election promising to negotiate a Labour-backed Brexit deal then putting that to a public vote.

Khan said this position “does not make sense” as the party risked losing leave-backing voters to the Tories and Brexit party anyway, while seeing many remain voters go to the Lib Dems. But he also said it was wrong for Labour to be swayed by the argument that some of its heartlands are pro-Brexit, saying the party should take a lead in the same way it has tackled racism in society, championed the NHS and fought for women’s rights.

Sadiq Khan photographed at City Hall, 10 October 2019. Photograph: Alicia Canter/The Guardian

“Can you imagine in the early 20th century Keir Hardie being told this women’s right to vote stuff is a bit tricksy on the doorstep in Doncaster and Wigan and I’m not sure we should be doing this,” he said. “My dad came to this country in the 1960s, and there were signs saying no blacks, no Irish, no dogs … Can you imagine if Labour leaders say: ‘In Wolverhampton there’s this poster that says: ‘If you want an N for a neighbour, vote Labour’, I’m not sure it’s electorally advantageous for us to run it past the Race Relations Act. Let’s not do it.’”

Asked whether a second referendum should come before a general election, Khan said: “Absolutely. Let’s resolve the issue of Brexit, give the British public a final say. Leave the EU on terms negotiated by the government or the option of remaining. Once that’s been resolved, have a general election as soon as possible and we’ve got such an exciting policy offer when it comes to the NHS, policing, schools, a Green New Deal.”

Khan’s position mirrors that of Tom Watson, the Labour deputy leader, and Emily Thornberry, the shadow foreign secretary, also revealed on Thursday she had concerns about an election before Brexit was settled. John McDonnell, the shadow chancellor, and Keir Starmer, the shadow Brexit secretary, are also understood to have raised concerns this week that stumbling into a quick general election could be a “trap” for Labour.

However, Corbyn has so far repeatedly made clear that he will vote for an election as soon as no-deal Brexit is off the table.

Khan said he was not necessarily worried about Labour trailing in the polls right now as they “go up and down” and the party gained ground during the campaign in 2017. But he said he was concerned about the scale of movement to the Lib Dems over Brexit at the European elections.

He dismissed Corbyn’s argument that the party should decide how to campaign only after Labour has negotiated its own Brexit deal, saying: “There is no Brexit that is better than remaining in the EU.”

Khan, who is up for re-election in May next year, said he had told Corbyn many times his own position and he believed there was further scope for the party to shift on its Brexit policy.

As well as the Tory candidate Shaun Bailey and the Lib Dem Siobhan Benita, Khan is facing the independent candidacy of the former Tory MP Rory Stewart. But he said he was also not concerned about the newcomer to the race, who has made a name for himself doing a series of walks throughout the country talking to voters.

“I walk all the time. What I don’t do is walk around London with an iPhone filming my walk,” he said. “And what I don’t do is treat Londoners like natives. We aren’t natives. We are people who enjoy walking around our city, whether it’s drinking in pubs, eating in cafes, eating in restaurants without being filmed. We don’t think there’s anything unique about that.”

He said he looked forward to debating with Stewart if he was still a candidate in six months’ time.

On climate emergency protesters Extinction Rebellion, who are halfway through their promised two-week period of action in central London, he said he feared they risked losing public sympathy .

“As somebody who is sympathetic to the cause, you’re in danger of losing public opinion. There is a tipping point where your actions can lead to people rather than being sympathetic to having animosity towards you. That’s why it’s a real dilemma. That’s why the way to win over public opinion and keep it is protest lawfully and peacefully.”

Asked if Extinction Rebellion had reached that tipping point, he said: “I’m not sure of the answer ... but as somebody sympathetic to the issue I do worry about public opinion not being won over but being lost.”