Failure to unite in the wake of the Labour leadership election could kill off the party, Sadiq Khan has warned.
The London mayor said Labour leader Jeremy Corbyn should drop plans to take revenge on his critics following his victory.
"We know from history - the Gang of Four in the 1980s - that when the Labour Party splits, we are out of power for a generation," Mr Khan told The Sunday Times.
"Now it is far more serious than that. If the Labour Party splits, it could be the end of the Labour Party."
Mr Corbyn confounded his critics by winning his second Labour leadership contest in two years, winning by a large margin of 313,000 votes compared to Owen Smith's193,000.
Speaking after his victory at Labour's annual conference in Liverpool, Mr Corbyn vowed to unite the party and claimed that he and Mr Smith were part of the "same Labour family".
He told The Observer that his empathic victory was a "vindication" and gave him a "very strong" mandate to lead.
"I have been given the authority by the members and that is what I intend to deliver on," he said.
With party membership increasing to more than 500,000 - and new recruits overwhelmingly backing his leadership - he added: "The participation is even higher, and my majority is bigger, and the mandate is very strong. So let's use it to reach out.
"With this huge membership, that has to be reflected much more in decision-making in the party."
