Unite leader Len McCluskey has been cleared of breaching union rules when he was re-elected.
He narrowly beat opponent Gerard Coyne in April 2017, but Mr Coyne had filed a series of complaints about the way the election took place.
The certification officer ruled: “None of the complaints succeed and they are therefore all dismissed.”
The trade union watchdog had already ruled in May that the election should not be re-run.
Mr Coyne had complained that the election should not have gone ahead as there was no genuine vacancy.
He has argued that Mr McCluskey resigned at a time that suited his bid for re-election, when he could have stayed in office until 2018.
The result of the Unite election in April 2017 saw Mr McCluskey win with 59,067 votes (45.4%) to Mr Coyne’s 53,544 (41.5%) on a turnout of just 12.2%. A third candidate, Ian Allinson, got 17,143 votes.
In the ruling on Friday, the certification officer dismissed a series of complaints, although it did find that Mr Coyne had in some ways been at a disadvantage in the race.
“The calling of an election a year earlier than anticipated by Gerard Coyne undoubtedly put him at a disadvantage as compared with Len McCluskey, who had a long-established track record in the post and a substantial and developed support network,” it said.
It also said the rules favoured the candidate with the most nominations and Mr Coyne’s lower profile nationally would have put him at a disadvantage.
However, it dismissed claims Mr McCluskey had breached the rules.
And it was critical of aspects of Mr Coyne’s campaign, including “misleading” information in some of the literature.
Unite’s assistant general secretary Gail Cartmail said it was “an emphatic ruling that ought to draw a line under matters once and for all, allowing this union to do what it does best, defend the interests of its members”.
Unite is Britain’s second biggest trade union and the Labour Party’s main financial backer.
The BBC understands that Mr Coyne will not be appealing against the ruling.
BBC political correspondent Iain Watson said it brought to an end what some saw as an 18-month proxy war over Labour’s future direction between Corbyn-supporting Mr McCluskey and Mr Coyne, who had backed the challenger, Owen Smith, in the 2016 Labour leadership contest.
He added that the dismissal of Mr Coyne’s complaints would not just cement Mr McCluskey’s position in Unite – but bolster his considerable influence in the Labour Party too.