London mayor Sadiq Khan has called for a second EU referendum, criticising the government's handling of Brexit negotiations with the EU.
Writing in the Observer, the Labour politician said that with the UK due to leave the EU in six months, it now faced either a "bad deal" or "no deal".
The debate had also become "more about Boris Johnson's political ambitions" than what was good for the UK, he said.
Tory minister Michael Gove said Mr Khan wanted to frustrate the will of voters.
The mayor's comments are also at odds with the Labour's official policy, which is to respect the outcome of the referendum but "leave all options on the table" if a deal is not agreed by Parliament.
Former Tooting MP Mr Khan said that although he campaigned to remain in the EU, he had accepted "the will of the British people was to leave the EU".
He said he had never expected to back calls for a second referendum, but had become "increasingly alarmed as the chaotic approach to the negotiations had become mired in confusion and deadlock".
With time running out for the British government to negotiate a final deal with the EU before March 2019, Mr Khan said the UK was left with two "incredibly risky" possibilities.
"Both these scenarios are a million miles from what was promised during the referendum campaign, only further exposing the lies and mistruths sold to the public," he wrote in the paper.
"I don't believe Theresa May has the mandate to gamble so flagrantly with the British economy and people's livelihoods."
'Not a rerun'
Appearing on BBC One's Andrew Marr Show, Mr Khan was shown comments he made to LBC radio after the Brexit referendum, in which he warned a fresh poll would lead to "even more cynicism" among voters.
He told Marr: "It's really important that this is not a re-run of the referendum but the British public having a say for the first time on the outcome."
Pushed on what voters should be asked in a fresh vote, he said it should be a choice between the government's deal and remaining in the EU.
Asked whether that would disenfranchise supporters of a hard Brexit, Mr Khan added: "My point is this. Rather than having a bad deal or a no deal, let's put that to the British public with the option of staying in the EU."
Mr Khan also dismissed suggestions he was trying to distract from his track record as London mayor, following criticisms of his record on knife crime, housing and transport.
Appearing on the same programme, Environment Secretary Mr Gove, a leading Brexiteer during the referendum, said Mr Khan's comments were "troubling".
"People voted clearly - 17.4m people - to leave the European Union," Mr Gove said. "Sadiq is essentially saying, 'Stop, lets delay the process, let's throw it into chaos'. And I think that would be a profound mistake."
Meanwhile, Lib Dem deputy leader Jo Swinson accused Labour leader Jeremy Corbyn of "backing the government every step of the way" over Brexit.
"Jeremy Corbyn is letting the prime minister off the hook," she said in a speech to her party's annual conference in Brighton.
"My message to him is very clear. Grow a backbone, stand up for the millions of people who voted for you and help us stop Brexit."
The Lib Dems want another referendum and are talking to disaffected anti-Brexit Labour and Tory MPs who, they say, are considering quitting their parties.
Earlier this month, Prime Minister Theresa May said the government would not back another vote.
Writing in the Sunday Telegraph, she said: "To ask the question all over again would be a gross betrayal of our democracy - and a betrayal of that trust."
Meanwhile, in a letter to Brexit Secretary Dominic Raab, published by The Sunday Times, shadow Brexit secretary Sir Keir Starmer warned the government that Labour MPs will vote down attempts to force the country into a "blind Brexit".
It comes as the People's Vote campaign, which wants a new referendum on Brexit, is attempting to change Labour Party policy, according to a leaked memo.
Earlier this month GMB general secretary Tim Roache said the union backed a referendum on the outcome of the Brexit negotiations, urging Labour to "follow suit".