“Red Len” – the hardline socialist and close ally of Labour leader Jeremy Corbyn – was understood to be losing support within public sector union Unite, following allegations about his conduct.
An official probe is studying claims that union rules were broken in the recent leadership poll which saw Mr McCluskey defeat rival Gerard Coyne for the general secretary post.
Allegations by Mr Coyne include suggestions union staff and facilities were used to support the re-election campaign in breach of electoral rules.
Sources said 12 complaints had reached former high court judge Jeffrey Burke, who is investigating claims arising from Mr Coyne’s appeal against the election result.
One union insider told the Daily Express: “This investigation is serious. McCluskey could be forced out.”
Another source said even some of Mr McCluskey’s most ardent supporters were disenchanted with him.
A Unite spokeswoman said last night: “These rumours have a habit of appearing every so often and there is never any substance to them.
“Twelve complaints have not been passed to the QC.”
Last month, leaked union documents revealed Mr McCluskey repaid more than £400,000 contributed by Unite towards buying his £700,000 London flat.
The union said the purchase agreement was an equity share arrangement that would be repaid 12 months after Mr McCluskey’s employment ended.
Mr McCluskey is one of the most influential union figures and Unite has been the Labour Party’s single biggest financial backer for years, donating £4.4million in the run-up to last year’s general election.
The union also helps fund the offices of dozens of MPs.