Len McCluskey, leader of Unite, was re-elected to his post after enjoying a narrow win over Gerard Coyne, who was the union’s regional officer for the West Midlands.
Mr Coyne, who was since been removed from his role, complained that Mr McCluskey had breached election rules by calling the contest, even though there was no vacancy for the job of general secretary.
He complained that the election was invalid, and that it should be rerun.
It resulted in an investigation by the certification officer, with a hearing in front of Jeffrey Burke QC.
The election, in April 2017, proved to be a bad-tempered and divisive campaign, with Mr McCluskey eventually winning by 59,067 votes to 53,544.
Mr Coyne has also filed nine other complaints about the conduct of the election, which will be the subject of another hearing, to be held in June.
Among Mr Coyne’s allegations are claims that union officials openly campaigned against him, that thousands of members were prevented from voting and that union resources were used to discredit him and support Mr McCluskey.
However, Sky News understands that none of these accusations would be sufficient to force a rerun of the election.
A Unite spokesperson said: “Unite welcomes the assistant certification officer’s rejection of Mr Coyne’s complaint and his ruling that the union acted in accordance with its rules in calling a general secretary election.
“It should be noted that when the election was called, no complaints or objections were made by Mr Coyne or any other individuals.
“Unite presumes that Mr Coyne will accept the judgement and looks forward to the election being fully upheld following June’s hearing.”