Labour is facing a furious internal row after party chiefs ruled that MPs hoping to become city mayors must promise to resign from the Commons if they are picked as mayoral candidates.

The surprise announcement came as a shock to Birmingham MP Gisela Stuart and Coventry MP Bob Ainsworth, who both hope to stand as the Labour candidate for mayor of their respective cities.

It means they now face a choice between giving up their ambition – or taking a massive gamble by pledging to give up their Parliamentary seats in order to fight a mayoral election they have no guarantee of winning.

Both MPs have contacted party officials seeking urgent clarification of the ruling.

The decision also means Labour leader Ed Miliband faces the prospect of a difficult by-election in Ms Stuart’s seat of Birmingham Edgbaston, which the party held with a majority of just 1,274 in 2010.

Labour could struggle to hold the seat without Ms Stuart as their candidate, as her maverick style and Euroscepticism is believed to have won over some constituents who might otherwise have voted Tory.

Another real possibility is that Ms Stuart could simply defy party bosses and refuse to quit the Commons.

A by-election might also be needed in Mr Ainsworth’s seat of Coventry North East, but this is a safe Labour seat which the party won with a majority of 11,775 in 2010.

Labour’s National Executive Committee has ruled that any MP selected as a the party’s candidate in a mayoral election or election for Police and Crime Commissioner must stand down from the Commons before polling day.

Referendums asking voters in major cities including Coventry and Birmingham whether they want to create a directly-elected mayor will be held on May 3.

If residents vote “yes” then Labour plans to send out ballot papers for activists to choose mayoral candidates on May 25 with the result, naming the candidates selected, to be declared on June 15.

The Labour ruling means that MPs who want to be considered as a candidate must promise that, if selected, they will quit the Commons before the mayoral election takes place on November 15.

It leaves the MPs in a double bind. First, they risk ending their political career if they leave the Commons and then fail to become mayor.

Secondly, it means they have to promise to abandon their constituents even to be considered as a candidate – when there is no guarantee of being selected.

In Ms Stuart’s case, she faces competition from former Erdington MP Sion Simon and former city council leader Sir Albert Bore, who also want the Labour nomination.

If she makes the pledge and fails to become Labour’s candidate she would not be required to stand down, but it would provide ammunition for Conservative opponents to use against her at the next general election.

Ms Stuart said: “I am asking for clarification about how exactly these rules will work and what the timing will be.”

Mr Ainsworth said he was asking for an explanation. He said: “I would like to know why this decision was taken and what the justification is.

“I am going to seek clarification because I don’t understand it.”

While Labour appeared keen on the idea of city mayors under Tony Blair’s leadership, the national party has been less enthusiastic recently.

Birmingham MP Jack Dromey (Lab Erdington) described the Government’s policy of forcing 11 major cities including Birmingham and Coventry to hold referendums as “Leninist”, as he represented Labour in the Commons in his role as a Shadow Local Government Minister.

Labour insists that it’s not opposed to referendums in principle, but believes they should only be held where local residents have demonstrated they want them.