You might have thought that Ken Livingstone would be on his best behaviour ahead of his 2012 rematch with Boris, but he's again shown why he'll never be part of the Labour establishment.

Ken has been openly campaigning for Lutfur Rahman, the independent set to run Labour close in the Tower Hamlets mayoral election. Under party rules, any member who campaigns for a non-Labour candidate is automatically expelled. Eight local candidates have already been kicked out over their support for Rahman.

The party rules state:

A member of the party who...supports any candidate who stands against an official Labour candidate, or publicly declares their intent to stand against a Labour candidate, shall automatically be ineligible to be or remain a party member.

As the Guardian's Dave Hill points out, Harriet Harman gave a fairly unambiguous reply when asked if action would be taken about Labour peer Lord Nazir Ahmed, another supporter of Rahman. She said:

There is nobody else that is a Labour candidate for Tower Hamlets, so basically people can either be supporting Abbas, which is what we want Labour supporters to do, and all of us in the Labour Party are doing, but if they are not supporting Councillor Abbas, if they are supporting somebody else, then they are opposing the Labour Party and you cannot be against a party and in it. So we think he's an excellent candidate. There must be no ambiguity about this. He is our candidate, and if you're a Labour Party member you cannot support a candidate from a different party.

The election is on Thursday and it's unlikely that any action will be taken but Labour activists are already questioning Ken's judgement. As I wrote yesterday, the chance to win control of the "Olympics borough" and its billion-pound budget makes this contest more significant than most by-elections. But the fact that the Tower Hamlets selection process was an unmitigated shambles will stand in his favour.

Rahman, the party's original candidate, was removed by Labour's National Executive Committeee after criticism of his alleged links to fundamentalist Islam and concerns over the "eligibility of participating voters". He was replaced with Helal Abbas (who submitted a dossier on Rahman to the NEC), a controversial choice, not least because he finished third, not second, in the original contest. Christine Shawcross, a member of Labour's NEC, suggested: "they put forward Abbas so as not to leave themselves open to the charge of deselecting a Bangladeshi and replacing him with a white man."

The political fallout from Ken's decision will likely depend on the result of Thursday's election. His endorsement is a late boost to Rahman's campaign and Labour admits the race is too close to call. But one thing is certain: Ken has already lost a significant amount of goodwill from grassroots members.

UPDATE: Ken's attempted to get himself off the hook by offering his support to Helal Abbas and calling for a second preference vote for Rahman. It should be enough to keep the NEC happy but the damage has been done.

Here's the statement in full: "I am disappointed by the way the NEC handled the selection in Tower Hamlets and I am sure that under Ed Miliband's leadership things would have been handled differently. However, my position is clear: I fully support Labour candidates in all elections and I am calling for Tower Hamlets residents to use their first preference vote for our candidate Helal Abbas.

"A second preference should be used for Lutfur Rahman to keep the Tories out."