A bid to oust Len McCluskey as the leader of the Unite union is to be launched later.
The trade union regulator will be asked to rule that Unite's recent general secretary election - which Mr McCluskey won by 5,500 votes - was invalid.
The defeated candidate in the contest, Gerard Coyne, says his supporters suffered bullying and intimidation.
A spokeswoman for Unite said its members would be deeply disappointed by the timing of the announcement.
'This isn't North Korea'
Mr Coyne's complaint is a challenge to one of Labour's most important power brokers days before the general election.
Mr Coyne says he was the victim of repeated harassment and interference by union employees loyal to Mr McCluskey.
Unite employees repeatedly breached guidelines that meant they should have been neutral in the contest to be general secretary, he said.
He told the Today Programme on BBC Radio 4: "The full weight of the union machine was used against me and as this is not North Korea, we are a democratic country and it's a democratic organisation, then I have the opportunity to call out that activity and the use of that machine to an independent arbiter."
The timing of the challenge was dictated by legal requirements, not the general election, he said.
But a Unite spokesman said: "Unite members will be deeply disappointed that Mr Coyne has chosen this critical moment in the fortunes of the labour movement to launch an unnecessary attack on his own union, something which can only help the floundering Tories."
Informing the media first was deplorable, the spokesman added, and Unite was confident the complaint had no merit and the union had conformed with the law.
Mr McCluskey previously said Mr Coyne's campaign amounted to a proxy war against Labour leader Jeremy Corbyn run by a cabal of hostile MPs.
He was re-elected to his position with a reduced majority in April.
Coyne was suspended as a regional officer shortly after voting closed in the general secretary election.
He is to lodge a formal complaint with the certification officer, who oversees trade unions. The officer has the power to order elections should be re-run.
Unite is hugely influential within the Labour party, as a major donor with places on the party's National Executive Committee and a key voice in policy making.