Ken Livingstone faced calls for his expulsion from the Labour Party today after he joined a rival candidate on the campaign trail.
The veteran, selected just last month as the party's candidate for the 2012 London mayoral race, appeared publicly with Lutfur Rahman.
Mr Rahman was sacked by Labour as their candidate in this week's election of a mayor in the borough of Tower Hamlets and is now standing as an independent.
Labour Party rules strictly forbid campaigning for anyone standing against a Labour candidate - with automatic expulsion set down as the sanction.
As many as eight local councillors are believed to have been suspended for backing Mr Rahman rather than the party's official candidate Helal Abbas.
Labour remained tight-lipped today over whether Mr Livingstone would face disciplinary action as it tried to focus attention on Thursday's election.
But Mr Livingstone told London's Evening Standard newspaper his appearance with the candidate did not amount to campaigning - and said keeping the Tories out was his main objective.
"I was not campaigning for anyone in particular. We've got to make certain we don't get a Tory as mayor of Tower Hamlets on Friday," he said.
"Rahman asked me to appear with him, Abbas hasn't. I haven't urged anyone to support anyone."
A Labour spokesman said: "He certainly came down to Tower Hamlets yesterday.
"But the bottom line is: Ken is our candidate for the London mayoralty and he supports all Labour candidates. He has made it clear he wants Labour's Abbas to win. That is what we are focusing on at the moment."
The actions of Mr Livingstone sparked fury among some Labour MPs however - with one publicly calling for his immediate expulsion.
Tom Harris said on his blog that, if Mr Livingstone had campaigned for a rival, "he should be expelled immediately, irrespective of whatever silly excuses he tries to come up with".
"What I suspect will actually happen is that Ken will mount his defence and the NEC (National Executive Committee) will bottle it."
Jim Fitzpatrick, the MP for Poplar and Limehouse who ran Oona King's unsuccessful bid against Mr Livingstone for the mayoral nomination, said the actions were "extremely disturbing" and should be investigated by party chiefs.
Greg Hands, the Tory MP for Chelsea and Fulham, said: "Ken Livingstone's blatant flouting of the Labour Party rulebook is a slap in the face to Labour activists across London and a direct challenge to Ed Miliband's authority.
"The longer his expulsion is delayed, the more convinced Labour Party activists will be that it's one rule for them, one rule for Ken."
Labour's rule book states that any member who "supports any candidate who stands against an official Labour candidate...shall automatically be ineligible to be or remain a party member".
Mr Livingstone will be well aware of the rules, having been expelled in 2000 under the same clause after deciding to stand - ultimately successfully - against official candidate Frank Dobson to be London's first directly-elected mayor.
He was controversially readmitted to the party in 2004, allowing him to be adopted as the Labour candidate and take City Hall for the party for the next four years.
In a statement, Mr Livingstone hit out at the deselection of Mr Rahman but insisted he "fully supported" the official candidate.
However, as the election was being held under the Alternative Vote system, he said, voters should be encouraged to give the axed candidate their second preferences.
"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," he said.
"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."