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.
PM Theresa May has said a second vote would be a "betrayal of our democracy".
The former Tooting MP 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."
In July, Mr Khan told the BBC's Andrew Marr Show he believed the British public should be consulted if Parliament voted against the government's final Brexit deal.
But on Sunday, the London mayor said he had now concluded people "must get a final say".
"This means a public vote on any Brexit deal obtained by the government, or a vote on a no-deal Brexit if one is not secured, alongside the option of staying in the EU," he said.
Earlier this month Mrs May said the government would not back another vote, writing in the Sunday Telegraph: "To ask the question all over again would be a gross betrayal of our democracy - and a betrayal of that trust."
However Mr Khan said the PM had "failed to negotiate a Brexit position with her own party - let alone agree a deal with the EU".
Making reference to Boris Johnson's resignation as foreign secretary over the government's Brexit strategy - dubbed the Chequers plan - the London mayor said the debate had become more about Mr Johnson's aspirations to become prime minister "than what's good for the country".
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".
The Labour Party's policy is to respect the outcome of the EU referendum in 2016, and not to call for a new one but to "leave all options on the table" if a deal is not agreed by Parliament.
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".