The counting of votes is underway in the Unite election after the ballot closed this week.
The aftermath of the voting period saw Corbynsceptic challenger Gerard Coyne suspended from his role as West Midlands regional secretary although he is thought to remain a union member.
The reasons for Coyne’s suspension have not yet been made clear.
Last year Coyne was subject to disciplinary action for addressing an Labour for the Common Good group of MPs.
Ian Allinson, the candidate to McCluskey’s left, is running a distant third, sources suggest. He received nominations from 76 branches.
As the biggest trade union in Britain, with 1.4m members, as well as Labour’s biggest donor, the election has the potential to reverberate beyond the organisation itself.
The result is due to be announced next Friday.