Sadiq Khan has called for a second referendum on Brexit before the country is taken down an "extremely damaging" path.
The Mayor of London warned that the UK was looking at the prospect of either a "bad deal" or a "no deal" Brexit, with the March deadline under 200 days away.
Mr Khan accused the government of adopting a "chaotic approach" to negotiations with the EU, and branded details of the agreement with the bloc so far as "vague".
Writing in The Observer on Sunday, Mr Khan said the British public should have the final say on what happens next.
"They [no deal and 'bad deal'] are both incredibly risky and I don't believe Theresa May has the mandate to gamble so flagrantly with the British economy and people's livelihoods," he wrote.
Mr Khan continued: "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."
Mr Khan's comments will add further pressure on Labour leader Jeremy Corbyn to state the party's stance on Brexit at the autumn conference later this month.
Many will hope that this leads Mr Corbyn to back a fresh Brexit poll.
Mr Khan wrote: "Terrifyingly, we are now in real danger of crashing out of the EU with no deal.
"Despite the fanciful assurances from [Boris] Johnson, this would be by far the worst outcome – with independent research showing that it could potentially result in 500,000 fewer jobs across Britain by 2030.
"These are real jobs and people’s living standards being put at risk.
"So, after a lot of careful consideration, I’ve decided the people must get a final say.
"This means a public vote on any deal or a vote on a no-deal, alongside the option of staying in the EU."
Meanwhile, Theresa May is facing strong opposition to her Chequers blueprint for leaving the EU from hardline Tory Brexiteers.
Without support from at least some opposition MPs, the government is likely to struggle to get any agreement through Parliament.
And shadow Brexit secretary Sir Keir Starmer has warned that Labour will reject any "vague" plans put before Parliament.
A loosely-worded deal could be struck that would be able to be sold to all sides.
Sir Keir, however, said the Government must meet its key Brexit tests, which include delivering the "exact same benefits" as the UK currently has as members of the single market and customs union, to win support from Labour.
In a letter to Mr Raab published by The Sunday Times, he added: "But, let me be equally clear: a vague political declaration would not meet those tests. Labour will not - and cannot - vote for a blind Brexit."