Keighley MP John Grogan, an ally of Mr Jarvis, criticised the ruling by Labour’s National Executive Committee made three days before the party announces its candidate to be South Yorkshire’s first directly-elected mayor.
Former soldier Mr Jarvis has said he would aim to combine the two roles since announcing his candidacy, but the NEC agreed at a meeting on Tuesday that MPs should not hold more than one full-time elected public office.
Mr Jarvis is currently battling Sheffield councillor Ben Curran for Labour’s nomination, which will be announced on Friday ahead of the election itself in May.
The decision has sparked a row within the party, as the Barnsley MP had made it clear prior to being shortlisted that he intended to perform both roles.
Mr Grogan told The Yorkshire Post: “I very much regret this decision. This is like a referee changing the offside rules in the 87th minute of a football match without consulting the Football Association. My understanding is that only Party Conference can change the rules.”
A Labour source said the party’s high command have been making clear to Mr Jarvis he cannot do both jobs since he announced his candidacy.