Gerard Coyne is understood to mulling taking action after narrowly losing out to the sitting general secretary in the leadership ballot.
Mr McCluskey secured 59,067 votes, while Mr Coyne took 53,544 and Ian Allinson came third with 17,143.
The turnout was just 12.2%.
Voting ended on Wednesday and the ballots were counted on Friday at the Electoral Reform Society.
Unite's acting general secretary Gail Cartmail said: "I congratulate Len McCluskey on his victory and would urge the entire union to pull together in the interests of our members, and not least to work for a Labour victory in the general election."
The contest between Mr McCluskey and Mr Coyne has been bitterly fought.
It has been described as a battle for the heart and soul of the Labour party, which Unite bankrolls with £1.5m a year.
Tony Burke, Unite's assistant general secretary, said: "I have fought in many union elections over 40 years - this was the vilest I have ever witnessed."
During the election Mr Coyne accused Mr McCluskey of being too close to Labour leader Jeremy Corbyn, while the Unite leader said his opponent was spreading "smears".
The two men also traded personal insults about each others living arrangements, with both accusing the other of being out of touch with union members.
On Thursday Mr Coyne was suspended from his role as Unite's West Midlands Regional Secretary, pending an investigation into "certain issues", according to an email sent to Unite members and staff in the area.
The union poll was scheduled to take place in 2018 but was bought forward by a year by Mr McCluskey to allow him to serve until 2020, when the next General Election was expected to be held.
When he launched his campaign last year he said he wanted to tackle attacks on jobs, pay and conditions for members.
He also threw his support behind Jeremy Corbyn and John McDonnell saying their policies were "closely aligned to ours" and saying he wanted to "work for a Labour government, to reverse austerity and start to re-invest in manufacturing and our communities."