Len McCluskey, leader of the super-union Unite, is to demand a Labour party investigation into what he claims is a plot by a “cabal” of MPs on the right of the party to undermine him and his close ally, leader Jeremy Corbyn.

In a move likely to open another chapter in the party’s civil war, McCluskey, who is standing for re-election as general secretary of the one million-member union, says rightwing Labour MPs and grandees, with the support of almost the entire media, have turned the election in his union into “a proxy war against Corbyn”.

McCluskey accuses the cabal, including deputy party leader Tom Watson, of “trying to abuse Unite’s democracy”. He says that, behind the scenes, the group has been backing his main rival, Gerard Coyne, whom he criticises for running “a shameful campaign full of lies, innuendoes and smears”. Asked who else was in the cabal, McCluskey named John Spellar, MP for Warley, Jack Dromey, MP for Birmingham Erdington, and Siôn Simon, MEP for the West Midlands and Labour candidate for the West Midlands mayoral election.

McCluskey’s comments come after fresh cracks emerged at the top of the Labour party on Friday, as Corbyn warned that the US missile strikes in Syria risked escalating the conflict, just hours after Watson had said they “appear to be a direct and proportionate response” to the Syrian regime’s use of chemical weapons.

The Unite leader said the union’s leadership contest had, in effect, been turned into a third leadership contest for Corbyn. “That is precisely what the media want it to be, and what the rightwing of the Labour party want it to be, and they have a puppet in Gerard Coyne that they can use,” McCluskey said.

“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.”

McCluskey is to ask the party to investigate after the union ballot is completed and the result announced on 21 April. “This will all be resolved by the union once this election is over. The union will be seeking answers from the Labour party on how this has been allowed to happen in this way,” McCluskey said.

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.”

Many Labour MPs see the challenge to McCluskey by Coyne, who is the union’s regional secretary in the West Midlands, as a chance to break the alliance between Corbyn and the Unite leader, and restore the party and its biggest paymaster more to the centre ground. Coyne accuses McCluskey of meddling too much in Westminster politics, rather than fighting for the everyday causes of Unite members. He has made clear he will be less involved in political activity if he replaces McCluskey and has suggested he will review how Unite, the biggest donor to Labour, channels financial support in future.

With Labour languishing well behind the Tories in recent opinion polls, victory for McCluskey, on the other hand, would shore up Corbyn’s position ahead of crucial local elections on 4 May in which Labour-controlled councils across Scotland, England and Wales could fall. McCluskey and Unite also hold several key position on Labour’s national executive committee.

Analysis of the local elections by the pollster Robert Hayward last week predicted the worst local election results for an opposition party since 1985, with Labour likely to lose its Scottish stronghold of Glasgow, and around 125 council seats across the country.

The Observer has also learned from Liberal Democrat and Labour sources that Tim Farron’s party is likely to run Labour closer than was expected in a byelection – also on 4 May – in the previously safe Labour seat of Manchester Gorton. Labour retained the seat at the 2015 general election, with a majority over the Tories of more than 24,000.

McCluskey said he was “cautiously optimistic” about retaining the post he has held since 2011. Relations between Corbyn and Watson have been strained since a coup attempt against the party leader after the Brexit referendum last year. Asked about Watson, McCluskey said: “People keep asking me what’s happened to Tom. I can’t really give you a proper answer. I don’t really know. We have not spoken since last July and it is sad that you lose friends but it is even sadder that Tom has turned on those of his friends who he relied upon in his darkest hours. He will have to answer to himself when he looks in the mirror every morning.”