Sadiq Khan has called for Jeremy Corbyn to back a second referendum before a general election.
The Mayor of London said the Labour leader would be wrong to continue with his current Brexit strategy, branding it a 'fudge'.
Mr Corbyn plans to demand an election within days of Prime Minister Boris Johnson requesting a Brexit extension from Brussels.
He has promised a second referendum on Brexit, but only after any Labour win at the polls.
Mr Khan suggested Mr Corbyn ballot party members over an immediate backing for Remain.
He told the Guardian: '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 Brexitnot being resolved and our position being so unclear; it's bad politics and the wrong place for us to be.'
Mr Khan said his comments had not been motivated by the mayoral elections in six months.
The Remainer said he felt compelled to point out when his party was making a mistake.
It comes as some Labour MPs and shadow cabinet ministers have put pressure on Mr Corbyn to support a second referendum before an election.
Today Remainer backbencher Jess Phillips said she may consider running for the party leadership if Mr Corbyn stands down.
The polls also show Labour behind as pro-remain voters appear to be shifting to the Lib Dems.
Mr Khan hit out at Labour's policy organised at the party's autumn conference three weeks ago.
During it members supported Mr Corbyn's plan to go into a general election with the pledge to get a Labour-back Brexit deal to be put to the pubic.
But the mayor said the move 'does not make sense' because the party's leave voters would shift to the Conservatives and Brexit Party anyway, and remainers would then flock to the Lib Dems.
He also said he disagreed with the idea Labour has pro-Brexit supporters in its heartlands.
On a second referendum before an election, Mr 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.'
He added after that there should be a general election straight away.
Mr Khan's position matches deputy leader Tom Watson's and shadow foreign secretary Emily Thornberry's, with the latter admitting on Thursday she had reservations about an election before Brexit.