Around 50 Labour MPs have signed a letter objecting to the decision to block Greater Manchester Mayor Andy Burnham from standing in a forthcoming parliamentary by-election, BBC News understands.
They have written to Sir Keir Starmer to warn the decision is a "real gift" to Reform UK, as polling suggests Burnham may be Labour's "very best chance at winning" the vacant Gorton and Denton seat.
The signatories added there was "no legitimate reason" to block Burnham and asked for Labour's ruling National Executive Committee to "re-evaluate" the decision.
Defending the move, Sir Keir said allowing Burnham to contest the seat would have diverted Labour's resources away from other important elections.
"Resources, whether that's money or people, need to be focused on the elections that we must have, not elections that we don't have to have," he said earlier.
