Dan Jarvis has been nominated as Labour's candidate to run for Sheffield City Region mayor in a move which will force him to quit as an MP or have a row with his party.
Announcing his nomination, the former soldier suggested he will attempt to defy a new Labour ruling and remain in his post as Barnsley Central MP in the event of his likely victory in the May 3 mayoral election, setting up a clash with the party's high command.
The party's National Executive Committee (NEC) this week passed a motion stating that MPs should not hold more than one full-time elected public office, which could force him to quit Parliament or drop out of the mayoral race.
But in what will be seen as a signal of his intent to continue as an MP, Mr Jarvis said he will need to "work with both local and national government" to negotiate a good deal first for the region, and then a wider Yorkshire-wide devolution deal, and stressed the result was "a vote of confidence in the platform on which I am standing".
A Labour source said "the rules are in place", although the motion is not formally part of the party rulebook, and that Mr Jarvis would have to stand down as an MP.
Mr Jarvis secured 2,584 votes from Labour members in South Yorkshire, beating Sheffield councillor Ben Curran's 1,903 on a turnout of 37% to win the nomination.
He has not commented directly on the NEC decision but his ally John Grogan has described it as "like a referee changing the offside rules in the 87th minute of match".
Mr Jarvis's nomination will raise hopes that a "One Yorkshire" devolution deal is a step closer.
In a statement on Twitter, the MP said: "Thank you to Labour Party members across South Yorkshire for selecting me to be their mayoral candidate.
"I am proud to have been chosen, grateful for the opportunity to service, and pleased to have been part of such a comradely contest; the conduct of our members has been in the best traditions of our Labour movement.
"The election of a mayor comes at a pivotal moment for the Sheffield City Region. To make the most of new opportunities, our first mayor will need to work with both local and national government to negotiate the best possible deal for the people of South Yorkshire.
"Only then will the mayor be able to end the status quo of how decisions are made and how public services are delivered; and use both devolution and cooperative principles to offer a more radical and effective way of serving the public.
"Today's result is a vote of confidence in the platform on which I am standing, and the potential of devolution; first in the Sheffield City Region and then across 'wider Yorkshire'."