Mary Turner, the long-standing president of the GMB union and Labour Party executive member, has died.
Labour leader Jeremy Corbyn said she had "paved the way for so many women in the union movement", while former GMB general secretary Paul Kenny described her as the most outstanding woman trade unionist of her generation.
The former dinner lady from London had been ill for years but continued in her post as president, chairing the GMB's annual conference in Plymouth last month.
Ms Turner received a CBE for her political service, particularly through the trade union movement, in recognition of her lifetime's work for trade union members throughout the UK.
Elected GMB national president in 1997, she was a union activist in the London borough of Brent for many years and represented the GMB on the National Executive Committee of the Labour Party.
Mr Corbyn said: "I'm sad to hear of the death of my friend Mary Turner. Mary was such a great campaigner for people.
"We first worked together in the early 1980s defending school meals in Barnet and I last saw her at a Labour national executive meeting.
"She paved the way for so many women in the union movement and encouraged others to speak out. I send my condolences to Mary's family and her extended family at the GMB union. RIP Mary."
Mr Kenny said: "Mary never lost her roots. She was hard working and treated everyone the same - from prime ministers to cleaners.
"She was a powerful figure in the Labour Party as well as the trade union movement."
GM general secretary Tim Roache added: "I and the whole of the GMB are truly heartbroken by the loss of our President Mary Turner, a woman who has been the heart and soul of our union for a generation.
"The word giant is sometimes overused but in the case of Mary she really was a true giant of our movement. Mention the name Mary Turner and people will smile and say she had that unique blend of fighting spirit mixed with care and compassion."
Born in Tipperary, Ms Turner was a trade unionist from the day she started work at the age of 16.
First joining the Tailor and Garment Workers Union, she worked at Jackson's Tailors on London's Oxford Street.
Colleagues described her as a trailblazer who never took no for an answer.
When she returned to work in 1970 after having children, she was a dinner lady in Brent and quickly set about organising the female workers who were poorly paid, untrained and treated badly.
She developed a lifelong passion that she would campaign on for decades to come - free school meals.
Having seen hungry children and the stigma of those who had to queue separately for free school meals, she ensured free school meals became Labour party policy.
A GMB statement said: "From feeding 600 young marchers during the people's March for Jobs in the 1980s, to fighting the National Front and leading, recruiting and supporting thousands of low paid workers, Mary has led our movement.
"In recognition of her incredible work, Mary was elected to GMB's executive where she served as the only woman out of 40 members."
She chaired the Labour Party in 2004. In 2010, Mary was awarded an MBE, which was followed by a CBE this year.