Controversial rail union boss Bob Crow has died, the RMT has confirmed.
In a statement, the union 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.
"The union’s offices will be closed for the rest of the day and the union will make further announcements in due course. The media have been asked to respect the privacy of Bob’s friends and family at this difficult and distressing time."
Millwall have also paid tribute to Mr Crow, a Lions supporter.
They said: "Millwall Football Club would like to extend our condolences to the family of Bob Crow who passed away on Monday night at the age of 52.
"Born in Shadwell, Bob was General Secretary of the National Union of Rail, Maritime and Transport Workers since 2002 and a lifelong Lions fan."
Mr Crow, who was 52, 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 today.
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.
"It was a privilege to campaign and fight alongside him because he never gave an inch."
UK Independence Party (Ukip) leader Nigel Farage tweeted: " Sad at the death of Bob Crow. I liked him and he also realised working-class people were having their chances damaged by the EU."
Mr Crow had been campaigning for the No2EU political party in May's European elections, arguing that workers were suffering because of policies from Europe.
In a statement, London Mayor Boris Johnson said: "I'm shocked. Bob Crow was a fighter and a man of character.
"Whatever our political differences, and there were many, this is tragic news.
"Bob fought tirelessly for his beliefs and for his members.
"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."
Sir Peter Hendy, London's Transport Commissioner, said: "We are shocked by this terribly sad and unexpected news. Our thoughts are with Bob Crow's family, friends and all those he represented."
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.
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.
"My thoughts are with his family, friends and colleagues in the RMT and wider union movement at this difficult time."