Labour mayor Jamie Driscoll says he has been "barred" from standing for the party as a candidate in the North East mayoral election.
North of Tyne mayor Jamie Driscoll called the decision "arbitrary and capricious".
Labour said "some applicants did not meet the threshold required" but did not explain why one of the favourites for the job had been omitted.
The North East Mayor position devolves power over transport, housing and skills across Northumberland, Tyne and Wear and County Durham.
Northumbria Police and Commissioner Kim McGuinness is still in consideration to stand as the party's candidate, alongside ex-MEP Paul Brannen and Newcastle City councillor Nicu Ion.
Former shadow chancellor John McDonnell said omitting Mr Driscoll from the list despite him being "widely credited with doing a great job" was "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," he said.
"There can be no other motive for excluding him."
The decision came in the wake of Sir Keir blocking Jeremy Corbyn from standing for the party in Islington North at the next general election.
Labour said the party holds "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," a statement said.
"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."
The election for the North East Mayor will take place in May 2024.
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