Keir Starmer previously told banned Labour mayor Jamie Driscoll that he wanted to "unify the party" before blocking him from standing, it has emerged.
In a friendly sit-down interview with Mr Driscoll during the 2020 Labour leadership contest Sir Keir told the North East mayor: "There is more that unites us than divides us".
But Mr Driscoll has now been blocked from standing at the next election – in a move critics say amounts to factional warfare by Sir Keir against the left.
Labour mayors Andy Burnham and Steve Rotheram, as well as the Unite trade union are among those who have criticised the decision and called for a re-think.
Mr Driscoll, who has been the North of Tyne mayor since 2019, recorded a 19 minute sit-down video with Sir Keir during his campaign to become Labour leader.
On the issue of factionalism, Sir Keir told Mr Discoll: "One of the most important things is unifying the party.
"That doesn’t mean everybody agreeing because you’re never going to get that in the Labour Party but it does mean creating an environment in which people can respectfully disagree and come together and make a decision and move forward."
He added: "If I lead the Labour Party and ask for unity, I’ve got to demonstrate that through my campaign and as leader of the Labour Party: because you can’t ask others to do something you’re not doing yourself."
Adding that "disciplining people to be united is going nowhere," he said: "That saying that 'there is more that unites us than divides us' is really powerful."
The interview was part of a charm offensive by Sir Keir to win the votes of Labour's left-leaning membership – who he has pivoted sharply away from since gaining power in the party. Critics say he ran on a false prospectus.
It was announced on Friday that Mr Discoll been blocked by the party from standing as its candidate for the mayoralty he already holds. The decision was taken by Labour's national executive committee, which is controlled by Sir Keir and his allies.
The Labour leader's faction on the right of the party has been accused of using its control of the NEC to exclude left-wing candidates from standing – to bolster their power in the party's elected positions. Sir Keir's allies say such decisions are often linked to allegations of antisemitism.
The party has not publicly said why Mr Driscoll has been barred from his role, but anonymous sources have linked it to Mr Driscoll once appearing on a panel with film director Ken Loach, who has himself been kicked out of the Labour party for supporting a proscribed organisation.
Without directly repeating the allegations, shadow business secretary Jonathan Reynolds on Monday told Times Radio: "Where a person has shared a platform with someone who themselves has been expelled from the Labour Party because of their position on antisemitism ... that would preclude them from going forward as a Labour candidate." There is no suggestion that Mr Driscoll agreed with Mr Loach's view on the matter.
Former shadow chancellor John McDonnell, who is from the left of the party, said: "To refuse to allow a serving mayor onto even a selection long list demonstrates that factionalism in the party is completely out of control.
"Jamie Driscoll is widely credited with doing a great job. There can be no other motive for excluding him.
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Show all 50Manchester and Liverpool regional mayors Mr Burnham and Mr Rotheram said: "We have worked closely with Jamie and seen first hand the good work he has done as Mayor.
"He has taken a constructive, non- partisan approach to his work, as shown by his success in delivering a new devolution deal with government for the North East. He has also been a valuable member of our MI0 group of Mayors.
"We believe he deserves to be treated with more respect than he has so far been shown."
Labour has not responded to a request for comment on the move to bar the north of Tyne mayor from the long-list.
Mr Driscoll said: "I’ve been barred from standing as North East Mayor by Labour. No explanation has been given.
"I’m proud to have created thousands of jobs, fought child poverty, built affordable homes and delivered our Green New Deal. I believe in democracy."
