A group of Labour MPs has described a decision by the party's ruling body which would prevent Dan Jarvis becoming the mayor of the Sheffield City Region if he stayed in parliament as "unfair" and "undemocratic".
The Barnsley Central MP won Labour's nomination to be candidate in May's mayoral election in a ballot last week.
Mr Jarvis was clear when he stood that, if he won, he would combine both roles.
However, Labour's National Executive Committee (NEC) ruled days before the ballot ended that these dual positions will no longer be allowed.
It is understood that the former Paratroop Regiment officer was given until 5pm on Tuesday to chose between the roles but he has written to the NEC outlining his position in detail.
Now, the Yorkshire and Humber Parliamentary Labour Group, which includes 36 MPs in the region, has written to John Cryer, chair of the Parliamentary Labour Party, asking the NEC to "reconsider their position as a matter of urgency".
The letter said: "Dan has been open from the beginning that if elected as mayor, he would continue as an MP. He was shortlisted on that basis and elected by the members of South Yorkshire on that clear understanding.
"For the NEC to change the rules at this stage and only days away from the close of the ballot is unfair, undemocratic and an insult to all members who have so recently taken part in the selection process.
"We also believe that this decision is almost certainly unconstitutional.
"It would be completely unacceptable if Dan were to be put in a position of not being able to be the candidate and if someone else were to be imposed.
"This is hardly an example of empowering and involving our membership in party decision-making."
Kevin Barron MP, chairman of the Yorkshire and Humber Parliamentary Labour Group, said on Twitter: "Absolutely ridiculous that we are having discussions about this now, Dan was clear when shortlisted and selected that he intended to stay on as an MP.
"You cannot change the rules of a selection process six weeks in, we should be campaigning in SY not dealing with this nonsense!"
The post of the Sheffield City Region Mayor has been mired in controversy because, unlike similar roles in Manchester and the West Midlands, no devolution and funding deal has yet been agreed to go with the appointment.
Two of the four South Yorkshire councils - Doncaster and Barnsley - have rejected a devolution solution centred on the county in favour of joining a pan-Yorkshire proposal.
Earlier this month, 18 of the 20 local authorities in Yorkshire agreed to back a proposal to the government to deliver a One Yorkshire devolution deal. Sheffield and Rotherham were the only Yorkshire councils not to sign up to the plan.
Mr Jarvis is the overwhelming favourite to win the contest as all of the 14 South Yorkshire constituencies which will take part in the election were won by Labour at the last general election.
He has not commented on the stand-off with the NEC.
Nominations for candidates for the May 3 election close on Friday, April 6.