London Mayor Sadiq Khan has said it is time for Jeremy Corbyn to be “braver” and start telling Leave voters they are “wrong” on Brexit.
Mr Khan was an early advocate for a fresh referendum, a position now backed by his party’s leader.
Yet there are frustrations with Labour’s attempts to appeal to both Remain and Leave voters, with critics voicing concern that the on-the-fence stance is losing them votes on both sides of the Brexit debate.
Mr Corbyn narrowly won a vote at conference last month, with members backing his policy of renegotiating a Brexit deal with the European Union and putting it to a public vote should he be victorious at the next election.
Remainers in the party had wanted Labour to ditch talk of supporting even a Labour-negotiated Brexit deal and come out in favour of staying in the European Union.
Mr Khan, in an interview with Italian newspaper La Repubblica, said it was time for Mr Corbyn to “provide leadership” on the issue and stop trying to be “all things to all people”.
He said, having seen the terms of Boris Johnson’s deal and analysis of a no-deal exit, that “all forms of Brexit are worse than remaining in the EU”.
A Remain stance would likely assist the former minister in his bid for re-election next year, given Londoners voted overwhelmingly to remain in Europe during the 2016 referendum.
“I’d like the Labour Party to be braver and provide leadership on this issue,” said Mr Khan, who will be vying for a second term at City Hall in May.
“Sometimes, saying to people who may want to leave the EU, ‘Listen, I respect you, but let me explain to you (why) I think you’re wrong and why I think we should campaign to remain in the EU.
“’Let me explain to you why the problems you’re having with your school, with your health care, with your children’s education, with housing, is not because of EU’.
“It’s in fact because we’ve failed or not done enough. That’s the conversation we should be having rather than trying to be all things to all people.”
Mr Khan said he recognised that his party leader had been on a “journey” when it came to Britain’s relationship with Europe, having for decades been a Eurosceptic.
But he added: “I think he should go a bit further and be unequivocally pro-Remain and explain to those who are Brexiteers why he disagrees with them and have the argument.”
On Labour’s switch to back a referendum, the former MP said: “Better late than never.”
In comments that are likely to reignite his long-running public battle with US President Donald Trump, who also is fighting to be re-elected in 2020, Mr Khan warned that the White House incumbent had become the “poster boy” for the far-right.
“My concern about Donald Trump is that he is actually the poster boy for far-right nativist populist movements around the world as a fact,” said the first Muslim mayor of a major Western capital.
“You look at people here in this country, like from Britain First or Tommy Robinson.
“All these people idolise and look up to Donald Trump. He’s the poster boy.
“The reason why we should be challenging Donald Trump is not because he’s been rude to me or saying bad things about me, but because what he stands for are policies that are now mainstream.”