Initial reports suggest Mr Crow died at Whipps Cross hospital, Leytonstone. Medics spent an hour trying to save his life. Some reports suggest Mr Crow had been unwell for some time.

The RMT confirmed reports that Mr Crow,  who had been leader of the union since 2002 and was involved in a number of high-profile disputes, had died.

The union made the announcement "with the deepest regret".

A brief statement 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 most recently involved in a controversial Tube worker's strike over plans to close ticket offices on the London Underground.

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Crow's trade union career saw him frequently at logger-heads with management and the government. He became assistant general secretary of the RMT in 1991, before being elected leader in 2002.

A former member of the Communist Party of Great Britain and the Socialist Labour party his hard left rhetoric was often criticised by those on the right and the centre left.

Mr Crow was regarded as a member of the  so-called “Awkward Squad” - the loose grouping of left-wing union leaders who came to power in a series of electoral victories beginning in 2002.

Former Mayor of London Ken Livingstone today led tributes to Bob Crow saying he fought for his members despite being demonised by the right wing press.

Speaking of his shock at the news, he said: "I assumed he would be at my funeral not me at his."

He told Sky News: "He fought really hard for his members. The only working-class people who still have well-paid jobs in London are his members."

He said Mr Crow was "broadly right on most key issues" and that if more people had fought for the conditions of the working classes "this country would be a much better place."

"With the passage of time people will come to see that people like Bob Crow did a very good job."

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."

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."

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."