Unite boss Len McCluskey has called for an investigation into a "cabal" of Labour MPs he claims are using the union's leadership election to fight a "proxy war" against Jeremy Corbyn.
Labour deputy leader Tom Watson is one of a group that that is trying to "abuse Unite's democracy", the general secretary told the Observer.
Mr McCluskey said West Midlands MPs were behind the campaign of his rival Gerard Coyne, who he denounced as a "puppet" and accused him of running "a shameful campaign full of lies, innuendoes and smears".
His supporters were "skilled masters of the darks arts" who have used their techniques in the Unite ballot, Mr McCluskey claimed.
Mr McCluskey said the vote was being turned into a fresh poll on Mr Corbyn's leadership, telling the newspaper: "That is precisely what the media want it to be, and what the right wing of the Labour Party want it to be, and they have a puppet in Gerard Coyne that they can use.
"Remember, those people who are running Coyne's campaign are skilled masters of the dark arts.
"It is in their DNA and though it is normal in political elections, it's quite unique that they have brought this into a trade union election."
Mr McCluskey named MPs John Spellar, and Jack Dromey and Sion Simon, MEP for the West Midlands and Labour candidate for the West Midlands mayoral election, as being part of the so-called cabal, the Observer said.
Mr McCluskey will ask Labour to launch a probe after the result of the general secretary vote has been announced on April 28 .
He added: "I am not seeking for the Labour Party to do any heavy-handed review. I am just bringing it to the attention of the party that things have happened that should not have happened."