London Mayor Sadiq Khan said he is "fed up" of Labour losing as he backed an inquest into the party's embarrassing election defeat.
A review set up by LabourTogether is set to analyse Labour's latest setback, with former Labour leader Ed Miliband set to take part.
Mr Khan has defended Mr Miliband's involvement, stating he is "one of many people who should be properly examining why we lost”.
“I’m fed up with losing. I don’t believe in heroic failure, we’ve lost four general elections now in a row," he said.
“I want the next leader to unite our party, unite our country, unite those who voted Leave with those who voted Remain, unite the North with the South, the old with the young.
“And that’s one of the reasons why it’s important not just Ed Miliband but many others look at why we lost and learn the right lessons.”
Giving his thoughts on the next leader, to replace Jeremy Corbyn, Mr Khan said they should not be chosen on the basis of their gender or the location of their constituency.
He said: “I think the idea that the next Labour leader should be chosen on the geography of their seat or their gender is wrong.
“I want to see the Labour leader chosen on the ideas he or she has, on their vision for our country and a proper analysis of why we lost.”
He would not be drawn on who he might support in a bid, stating he was keeping an open mind.
His comments came as Labour MP Dan Jarvis hinted he could join the list of candidates running to replace Mr Corbyn as party leader.
Mr Jarvis, a former officer in the Parachute Regiment who is also Sheffield City Region mayor, said he cared deeply about the party and wanted to “play my part” in its future.
He delivered a scathing verdict on Mr Corbyn’s leadership and said voters in the party’s traditional heartlands like South Yorkshire felt Labour no longer spoke for them.
Mr Jarvis, who is MP for Barnsley Central, indicated he would reflect over the Christmas break on whether to run for the leadership himself.
Asked whether he could stand, he said: “I will be slightly suspicious of anybody who is declaring their candidacy on Christmas Eve.
“I have a very, very significant commitment as mayor. I take that very seriously, but I do want to play my part. I care deeply about the Labour Party, care deeply about the country.”
Elsewhere, another potential leadership contender, Lisa Nandy, warned the party is in danger of learning the wrong lessons from the election.
She expressed concern about a controversial review, including former leader Ed Miliband, into its performance set up by the Labour Together group.
The Wigan MP said that although she was part of Labour Together she had only learned of the review two days ago.
