Labour's candidate for London mayor, Ken Livingstone, has been campaigning with a non-Labour candidate in a separate local election in the East End, in breach of his party's rules.
He backed Lutfur Rahman, an independent running to be mayor of Tower Hamlets.
Labour rules state that any member who campaigns for another party should be automatically expelled.
Mr Rahman ran as an independent after being deselected as Labour's candidate over "serious allegations" against him.
These concerned both "the eligibility of participating voters" and Mr Rahman's "conduct", Labour said in a statement.
The party's ruling body, the National Executive Committee (NEC), took the decision last month, even though Mr Rahman was voted in by local members.
And Labour has already suspended eight local councillors for supporting Mr Rahman, BBC London political correspondent Karl Mercer said.
'Moment of madness'
Mr Livingstone said Labour had made "a big mistake" in removing Mr Rahman as its candidate.
"It took five years to put the Labour Party back together after George Galloway defeated Oona King down here," the ex-mayor said.
"All that was blown away in a moment of madness by Labour's NEC."
Labour's rules state that "a member of the party who supports any candidate who stands against an official Labour candidate shall automatically be ineligible to be, or remain, a party member".
Labour would not comment on whether it would take any action against Mr Livingstone.
The five candidates standing to be mayor of Tower Hamlets on Thursday are:
- Helal Uddin Abbas, Labour Party
- Alan Duffell, Green Party
- John David Macleod Griffiths, Liberal Democrats
- Neil Anthony King, Conservative Party
- Lutfur Rahman, Independent