Len McCluskey overwhelmingly won the contest to become new leader of the Unite union on Sunday.
Mr McCluskey, one of Unite's assistant general secretaries, gained 101,000 votes to easily beat off his three rivals for the job Jerry Hicks, Les Bayliss and Gail Cartmail.
"I am honoured and humbled at the confidence Unite members have shown in me," Mr McCluskey said after the final count was announced.
"My first task now will be to bring our union together and unite it in a campaign against the devastation the government is unleashing against working people and their communities throughout the land."
His victory will be confirmed officially on Wednesday.
Mr Hicks, a lay member of the union, came second with 52,000 votes, beating Mr Bayliss into third with 46,000 votes and Ms Cartmail into fourth with 39,000 votes.
She said she was "really pleased" with the level of support she achieved given that no organised group within the union had backed her.
"I think this shows I ran a very strong campaign," she said.
Mr McCluskey will take over next year from Tony Woodley and Derek Simpson, who have served as joint general secretaries since May 2007 when Unite was formed by the merger of Amicus and the Transport and General Workers Union (T&G).
A Labour Party member for 39 years, Mr McCluskey is a lifelong trade union activist and has held several senior posts with Unite and the T&G.
He began work as a docker in his native Liverpool in the 1960s, quickly becoming a senior shop steward.
In 1979 he was appointed an officer of the T&G, as well as representing members in a broad range of trades and industries.
He played a leading role in the '80s campaigning in his community against the attacks of the then Tory government led by Margaret Thatcher.
In 1990, Mr McCluskey became national secretary for the general workers group, leading negotiations in a diverse group of sectors including building materials, paper and packaging, building services and process industries.
He also led the white-collar group for a three-year period.
In 2004 he was promoted to become the union's national organiser for all the service industries, including public services, construction and the voluntary sector.
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