Len McCluskey has been re-elected as general secretary of Unite after beating his nearest rival by over 5,500 votes.
Mr McCluskey polled 59,067 votes against 53,544 for Gerard Coyne and 17,143 for Ian Allinson.
The turnout was 12.2%.
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 turnout in this important election can give no cause for satisfaction and, while the tone of the campaign will not have helped, the underlying reason remains the archaic and expensive balloting system imposed on trade unions by law.
“The sooner we can more to secure and secret workplace and online voting the better for union democracy.”
Mr Coyne was surprisingly suspended from his post as the union’s West Midlands Regional Secretary on Thursday.
An email was sent to Unite members and staff in the West Midlands informing them that Mr Coyne had been suspended from his job pending an investigation into “certain issues.”
The month-long campaign was one of the most bitter ever experienced in a leading trade union, with Mr Coyne accusing Mr McCluskey of being too close to Labour leader Jeremy Corbyn, with the Unite leader hitting back, accusing his rival of pedalling “smears”.
Tony Burke, Unite’s assistant general secretary, said he was pleased with the result, adding: “I have fought in many union elections over 40 years – this was the vilest I have ever witnessed.”
Gerard Coyne said it had been a “very close” count and the result sent some “very serious” messages to Unite.
“I am proud to have run a campaign that faced up to the issues that concerned members. Unite needs to change, and it needs to put its focus back where it belongs, on looking after the real interests of the members of the union.
“It’s been a hard and robust campaign. The union machine consistently attempted to bully and intimidate me, something that has continued even after the close of polls.
“Nevertheless tens of thousands of members backed my fight to change our union for the better.
“On the downside, turnout has fallen disastrously. Many members have reported to me that they did not get their ballot paper at all or if they did, that it arrived literally on the day polls closed and so was useless.
“This was no vote of confidence, with falling turnout and a halving of Len McCluskey’s previous vote. It’s time for all those that were involved to reflect on the message that the union’s membership are sending to the organisation.”
Ian Allinson said he had secured a “respectable” vote, adding: “We put important arguments into the union, made it harder for Coyne to drag the debate to the right, prevented him hoovering up all discontent, showed that it was possible to run a clean campaign, and connected up many members who want to see a stronger union.”