Rail union leader Bob Crow has died, it has been announced.

The Rail, Maritime and Transport 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, 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.''

Bob Crow's death shocked his fellow union leaders, and drew tributes from his adversaries as well.

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.

''He was always a good friend and comrade to me. We will miss him, and our thoughts are with his family and the RMT at this difficult time.''

Matt Wrack, leader of the Fire Brigades Union, said: ''Bob's passing is very sad news and the thoughts of firefighters will be with his family.

''Bob was a good friend to me personally and to the Fire Brigades Union as a whole.

''He was a strong leader for the labour movement and he'll be sorely missed by those who knew him.''

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.''

Manuel Cortes, leader of the TSSA rail union, 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.''

Network Rail chief executive, Mark Carne, said: ''On behalf of everyone at Network Rail, I would like to offer my sincere condolences to Bob Crow's friends and family, as well as the many thousands of RMT members on whose behalf he worked so tirelessly.

''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.''

Grahame Smith, Scottish TUC general secretary, said: ''Everyone at STUC is deeply saddened by the passing of Bob Crow. Since 2002, Bob led his union with courage and distinction and will be greatly missed, not just by his own members but by trade unionists across Scotland and the whole of Britain.

''STUC echoes the request of RMT that everyone should respect the privacy of the friends and family of Bob Crow at this distressing time.''

Former mayor of London Ken Livingstone said Mr Crow fought for his members despite being demonised by the right wing press.

''He fought really hard for his members. The only working-class people who still have well-paid jobs in London are his members.''