The trade union movement is in shock after the sudden death of firebrand rail union leader Bob Crow at the age of 52.
The general secretary of the Rail, Maritime and Transport union is believed to have suffered a heart attack at his home in east London.
The union announced the news "with the deepest regret" and said his death would leave a "massive gap".
Tributes poured in from unions, politicians and campaign groups, as well as business and industry leaders.
London Mayor Boris Johnson, who was embroiled in a bitter dispute with Mr Crow over Tube ticket office closures last month, said Mr Crow had fought "tirelessly" for his members.
The union revealed the news in a brief statement which said: "It is with the deepest regret that RMT has to confirm that our general secretary Bob Crow sadly passed away in the early hours of this morning.
"RMT would request that all media respect the privacy of the friends and family of Bob Crow at this distressing time."
Mr Crow was one of the most high-profile, left-wing union leaders of his generation, sparking as much anger from passengers hit by rail and Tube strikes, as praise from his members for winning pay rises.
He was constantly involved in industrial disputes and campaigns and led a walkout by London Underground workers last month in a dispute over ticket office closures.
The straight-talking south Londoner was a passionate supporter of Millwall Football Club.
His death caused shockwaves in the trade union movement, and there is no obvious candidate to take over his job
Manuel Cortes, leader of the TSSA rail union, who stood on picket lines with Mr Crow during last month's Tube strike, said: "Bob Crow was admired by his members and feared by employers, which is exactly how he liked it."
London's mayor spoke for many when he expressed shock, saying: "Whatev er our political differences, and there were many, this is tragic news.
"There can be absolutely no doubt that he played a big part in the success of the Tube, and he shared my goal to make transport in London an even greater success. It's a sad day."
Under Mr Crow's leadership, membership of the RMT increased by more than 20,000 to 80,000, embracing workers ranging from seafarers and rail staff to cleaners.
He spoke at rallies and meetings most weekends, and was always in demand to support campaigns.
He became a target of right-wing commentators, who criticised his militancy and involvement in disputes.
He even had to explain why he had gone on holiday in the run-up to last month's London Underground strike.
Mr Crow was campaigning for the NO2EU party and planned to stand in May's Euro elections, arguing that workers in the UK were being hit by EU policies.
The RMT parted company with Labour years ago after disagreements over policies, although the union continued to send its affiliation fee to the party, only to have its cheques returned.
Labour leader Ed Miliband said: "Bob Crow was a major figure in the labour movement and was loved and deeply respected by his members.
"I didn't always agree with him politically but I always respected his tireless commitment to fighting for the men and women in his union. He did what he was elected to do, was not afraid of controversy and was always out supporting his members across the country.
"He was a passionate defender of and campaigner for safe, affordable public transport and was a lifelong anti-fascist activist."
Mr Crow's older brother, Richard Crow, said he believed the union leader had suffered a heart attack in the early hours of today.
Speaking to Sky News, he paid tribute to the "loveable little rogue". Richard, whose father was also a trade union leader, said: " It was about 7am that I got the call (from my sister). I presume some time in the night he had some problems. We're really trying to find out exactly what happened.
"We grew up together in Chigwell in Essex and he was a very likeable chap - no matter what people said about his politics.
"He was honest, he looked after the people he was supposed to look after, and he was a great man as far as honesty and beliefs went.
"He was a person who believed in justice."
Richard, who is three years older than his brother, said: "People moaned that he lived in a council house, that he never drove a car - he lived a life of the average guy in the street and that's a rare thing these days.
"When people have a high office in life they fall for the big trappings of the flash cars and the big hotels and big houses. But Bob wasn't like that, he was a genuine person of the people.
"He was one of those loveable little rogues, one of those guys that had bundles of friends."
TUC general secretary Frances O'Grady said: "This is shocking news. Bob was an outstanding trade unionist, who tirelessly fought for his members, his industry and the wider trade union movement."
Network Rail chief executive, Mark Carne, said: " Bob possessed a deep understanding of the rail industry and his contribution to its success was significant, in particular the focus he gave to working with Network Rail on improving passenger and workforce safety."
Transport Secretary Patrick McLoughlin said: " Like many, I will remember him as a passionate voice for safety on the railways and the wellbeing of those who work on them.
"While we may not always have agreed on how to run our railways, he was a powerful advocate who led his organisation from the front and made an important contribution to the debate around the future of rail services in this country."
He said Mr Crow was always "very straight" with his dealings with the Government.
Len McCluskey, general secretary of Unite, said: "Bob was a life-long, and highly successful fighter for the interests of his members and for working people as a whole. I am sure that is the only epitaph he would have wanted."
It is understood that Millwall Football Club will include a tribute to Mr Crow in its programme for its game against Charlton on Saturday.