A supporter of Sir Ken Jackson, Downing Street's closest union ally, has resigned over an alleged attempt to cover up a ballot rigging scandal.
Roger Maskell, an official in the Amicus union, quit after he was accused of an "amateurish" attempt to wipe incriminating computer records.
The London and south-east regional secretary of the union's AEEU engineering section stood down following complaints from Sir Ken's opponent, leftwinger Derek Simpson, in an increasingly bitter election.
Mr Maskell had been confronted with evidence that he tried to destroy proof of a "flying voters" scam exposed by the Guardian in April.
Electronic records, showing at least four officials had been switched between branches to vote for Sir Ken at more than one nomination meeting each, were found to have been tampered with.
Sir Ken, seeking to stay on as joint general secretary of Amicus despite reaching 65, hopes the resignation will draw a line under a controversy that has marred his re-election campaign.
An Amicus spokesman said: "We said we took a very dim view of any vote rigging allegations. Roger Maskell has admitted he acted in a way which was not proper for his position and has offered his resignation."
Ballot papers go out later this month and Labour's high command this week signalled support for the head of a union which donated £2m to the party's 2001 election campaign.
The chancellor, Gordon Brown, effectively endorsed Sir Ken on Monday over lunch at an Amicus-AEEU conference in Blackpool, while Mr Blair repeatedly praised him in a video made by the union to distribute to activists.