Sharon Graham has won the election to be the next general secretary of Unite after receiving almost 5,000 more votes than her nearest rival.
Sources said she received 46,696 votes, with 41,833 for Steve Turner and 35,334 for Gerard Coyne.
Ms Graham will take over from Len McCluskey as leader of one the country’s most powerful trade unions.
Although the official announcement will be made later on Wednesday, Labour leader Sir Keir Starmer tweeted his congratulations, writing: “I’m looking forward to working together to improve the lives of working people across the country.”
Ms Graham leads Unite’s Organising and Leverage Department, which specialises in taking on hostile employers.
She has led recent disputes at British Airways and Crossrail as well as campaigning to unionise Amazon.
She described herself as the “workers’ candidate”, pledging to take Unite “back to the workplace”.
She has said she wants to rebuild the union as a movement which delivers industrially and politically.
Unite members at the workplace want real change, not a settling of old scores or a Westminster rematch
Speaking about the election campaign before the result was announced, she said: “My slogan all along has been ‘Back to the workplace’ to build the union to fight for jobs, wages and conditions.
“That has been rewarded with huge support for me in big industrial branches like Hinkley Point, London construction, Ireland construction and Vauxhall.
“My campaign co-ordinators estimate that my branch nominations cover a potential voting base of 250,000 members – and that’s before the 300,000 women in the union have had their say.
“I am not a member of any Unite or Labour faction – other than my own supporters group.
“Unite members at the workplace want real change, not a settling of old scores or a Westminster rematch.”
“But I know that she will build on our values and harness the talents of our great union in the service of our members and our movement.”