Sadiq Khan has said Brexit could be stopped if the Labour Party's next election manifesto committed to staying in the EU or to holding a second referendum.
The mayor of London reportedly made the comments after telling a delegation of EU ambassadors he hoped the EU flag would stay flying outside City Hall forever.
Speaking to the Guardian on whether he thought Brexit could be avoided, he said: "Yes. I'm an optimist. For it to have credibility with the British public, there would have to be a Labour manifesto offer, because the public would say, not unreasonably, 'Hold on a sec, we voted to leave and you're now sticking two fingers up at us.'"
He continued: "You'd have to spell out, in black and white, what we'd do if we won the general election. What could trump the referendum result is us having a manifesto offer saying, we would not leave the EU, or we would have a second referendum."
Khan's meeting with EU ambassadors at City Hall saw him accept that it was a pipe dream for a post-EU London to continue doing business in the same way.
"It's illogical to assume we can have a better deal or as good a deal with the EU outside the EU. It wouldn't happen with a tennis club, or any club," he is reported as telling them.
He also told the delegation the EU's flag currently flies outside City Hall and that he wishes it could do so forever.
His comments come as Labour has been accused of giving mixed messages on Brexit.
Party leader Jeremy Corbyn insisted the party supported leaving the EU single market, but shadow Brexit secretary Sir Keir Starmer claimed nothing should be off the table – an approach echoed by the shadow Home Secretary Diane Abbott.