Gisela Stuart’s hopes of becoming Birmingham’s first elected mayor have been dealt a potentially fatal blow by Labour party officials.

The National Executive Committee has decided that any Labour MP selected to run for mayor must resign from the Commons and trigger a by-election, raising the possibility of an unwanted contest in Mrs Stuart’s Edgbaston seat.

The ruling could also be significant for Liam Byrne, the MP for Hodge Hill, who is also said to be considering entering the selection race to become Labour’s mayoral candidate in Birmingham.

While a by-election in Hodge Hill might not cause Labour too many problems, Edgbaston is far more marginal and was a safe Conservative seat until 1997 when Mrs Stuart won for Labour. She has defended the seat three times since then, but a by-election in mid Parliamentary term could cause problems and would undoubtedly be seen as a test of Ed Miliband’s leadership of the Labour Party.

It’s though that the prospect of an unwanted by-election could weigh heavily in the minds of party members, who might shy away from choosing Mrs Stuart as a result.

The NEC’s decision would appear to deliver a huge boost to the hopes of Sion Simon, who stood down as the MP for Erdington at the 2010 General Election in order to campaign to be Labour’s mayoral candidate.

If the referendum on May 3 results in a vote in favour of Birmingham having an elected mayor, Labour will move to shortlist candidates on May 16. Ballot papers will go out to Birmingham party members on May 25. Hustings meetings must be concluded by June 10 and the ballot will close on June 13, with the result declared on June 15.

The same ballot dates will apply for Police Commissioner selections, giving successful mayoral and commissioner candidates exactly five months to campaign before joint elections are held on November 15.