Len McCluskey is being accused of "panic and desperation" after his rival in the bitter Unite leadership election was suspended from his post in the union.
Supporters of Gerard Coyne, who is bidding to replace Mr McCluskey - known as "Red Len" - as general secretary, claim the leader and his supporters fear defeat or a close result.
The election has been one of the most bitter in the trade union movement in recent years, with both candidates trading insults about the lifestyle and conduct of their opponent.
Mr Coyne has attacked Mr McCluskey over a £400,000 loan from the union to buy a "swanky" £700,000 penthouse flat in the shadow of the Shard close to the River Thames in London.
Mr McCluskey hit back, accusing Mr Coyne of living in a "million pound mansion in the countryside" in Worcestershire which is beyond the reach of blue-collar Unite members.
The suspension came less than 24 hours after the end of voting in the election, as counting of the 140,000 votes cast is about to get under way prior to an announcement of the result next week.
Sky News understands that the decision to suspend Mr Coyne from his post as West Midlands regional secretary was taken by the acting general secretary Gail Cartmell, a McCluskey ally.
"They can only suspend him as a union official," a leading Coyne supporter told Sky News. "To suspend him from the union they'd have to go through the proper procedures.
"It smacks of panic and desperation. They feel they might have lost or it's very close. There are suggestions that it's too close to call. We'll have to wait and see if it's valid or not."
The move is the latest bizarre twist in the acrimonious fight for the Unite leadership, which has been described as a battle for the heart and soul of the Labour Party.
The contest is crucial to Labour's future because left-winger Mr McCluskey is Jeremy Corbyn's chief trade union cheerleader and Unite bankrolls the party with £1.5m a year.
Mr Coyne, who has branded Mr McCluskey "Jeremy Corbyn's puppet master", is backed by Labour MPs who want to get rid of Mr Corbyn and is also supported by Labour's deputy leader Tom Watson.
The latest shock move in an election battle that has become a soap opera, with smears directed against both candidates and allegations of dirty tricks, was announced by Mr Coyne's spokesman.
"We can confirm that Gerard has been suspended from his position as West Midlands regional secretary of Unite the Union," he said. "We will not be commenting further at this stage."
Asked to explain the reasons for the suspension, a Unite spokesman told Sky News; "We've got no comment to make on that, I'm afraid."
But Unite members in the West Midlands have been sent an email by union official Des Quinn, in which he told them: "This is to advise that Gerard Coyne has been suspended from his duties as regional secretary pending an investigation into certain issues which have arisen.
"The investigation is, of course, being conducted under procedure, and it should not be assumed that any offence has necessarily been committed. You will appreciate that it is not possible to comment further on this matter."