Selection contests are often nastier than elections themselves, and the mayoral battle is a good example.

It remains to be seen whether Birmingham will choose to have an elected mayor once the referendum takes place on May 3. There’s a lot of debate about what will happen if the city votes for change, which is understandable, but nobody’s taking the result for granted.

However, if voters do say that they want a mayor then Labour will swiftly begin the process of selecting a candidate. And that battle, before we even know whether there is a job to fight over, is already becoming nasty.

Suggestions that sitting MPs should be barred from standing do appear to be motivated by a fear that a series of by-elections across the country could be disastrous for the party.

After all, there’s nothing for Ed Miliband to gain by holding on to seats. In the mid-term of a government which has made all sorts of unpopular decisions, the major opposition party should be popular.

So he could hold on to five Labour seats and the nation would shrug its shoulders. Lose the sixth seat, to Respect or the Tories and it would look like a disaster.

Nonetheless, another theory doing the rounds, and not just at Westminster, is that the proposed ban is a neat trick by Sion Simon and his supporters – including the influential Black Country MP Tom Watson, Labour’s Deputy Chair – to hand Sion the candidacy by simply stopping his rivals, Birmingham MPs Liam Byrne and Gisela Stuart, from standing.

Mr Simon has already quit the Commons, so a ban on sitting MPs would keep his opponents out of the race and allow him to become Labour’s candidate by default.

In an effort to disprove the conspiracy theories, Mr Simon has penned an article for today’s Post specifically opposing a ban. But there’s a real sting in the tail, as he suggests that if any sitting MP is selected as a mayoral candidate, they could be asked to pay the £30,000 plus cost of a by-election.

He also wants them to promise not to stand as an independent, presumably to stop them following in the footsteps of Ken Livingstone who turned his back on Labour and stood against the official party candidate in the London mayoral contest in the year 2000 – and won.