The London mayor has heaped pressure on his party leader by breaking rank and backing a second referendum.
Sadiq Khan, who initially ruled out a second vote, now believes the government should go back to the people over the Brexit deal.
He thinks Britain is heading for either a bad deal or no deal.
Writing in the Observer he said: ‘They 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 blamed the government’s performance on his U-turn citing a threat to living standards and jobs.
He added: ‘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 writes. ‘People didn’t vote to leave the EU to make themselves poorer, to watch their businesses suffer, to have NHS wards understaffed, to see the police preparing for civil unrest or for our national security to be put at risk if our cooperation with the EU in the fight against terrorism is weakened.’
Speaking on the Andrew Marr Show he added: ‘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.’
The major intervention by a leading Labour figure increases the pressure on Jeremy Corbyn to shift his party’s stance on Brexit.
Khan’s comments come as shadow Brexit secretary Keir Starmer confirmed his party would vote down any attempts to force the country into a ‘blind Brexit’.
He warned government counterpart Dominic Raab the opposition would reject any ‘vague’ plans put before parliament.