Labour is facing a backlash after a sitting left-wing Mayor was blocked from standing for the party in a new role representing the North East.
Jamie Driscoll, The North of Tyne Mayor since 2019, described the decision not to allow him to run in the mayoralty contest as "arbitrary and capricious".
A senior Labour source said the move was linked to an on-stage appearance with director Ken Loach, who was expelled from the party in 2021.
"He then refused to apologise," the source said. "Those decisions have consequences - namely, it was impossible for him to go forward as a Labour candidate".
But Mr Driscoll said on Friday evening he had been given no explanation for being "barred" from running in the contest to become the first North East Mayor.
He added: "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."
He later told the BBC : "The decision itself is capricious. I am not sure what the foundations [are] of any of this - and perhaps it is, that simply they do not like my politics.
"I do stand on picket lines, I was on a picket line this morning with workers who are in dispute. Perhaps that's it, but honestly I do not know, they have not told me why."
The ex-Labour leader Jeremy Corbyn, who has been blocked from standing for the party at the next general election, expressed his solidarity with Mr Driscoll.
Left-wing MPs still in the party also voiced outrage over the "shocking" decision.
The MP for Norwich South Clive Lewis said: "I didn't think it was any longer possible to be shocked by such sectarian behaviour. But here I am, jaw on the floor."
He described Mr Driscoll as "one of our most dynamic, successful" Labour mayors, adding: "Such a toxic political culture just isn't conducive to good government".
Ex-Shadow Chancellor John McDonnell added: "This is staggering news. 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"
A Labour Party spokesperson said: "The North East Mayoralty is a unique opportunity for the people of the North East to take more control over the way our region is governed, with powers over housing, education, skills, transport and so much more.
"The Labour Party holds its candidates to a very high standard. During this process, some applicants did not meet the threshold required to proceed to the longlist stage. We do not comment on individual applications.
"Local members now have a fantastic longlist of candidates from which they will choose the Labour Party’s candidate to be the very first North East Mayor."
