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Tim O'Toole, chief executive of FirstGroup and former London Underground managing director, said he was "shocked and saddened" by the news of Bob Crow's death.
Mr O'Toole said:
Bob Crow's brother said he believed the union leader suffered a heart attack in the early hours of today and paid tribute to the "loveable little rogue".
Mr Crow's older brother, Richard Crow, told Sky News: "It's very sad. 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."
When asked what Bob was like when he was younger, Richard replied: "He was one of those loveable little rogues, one of those guys that had bundles of friends. He would be a cheeky chappie Cockney kid. He was a lovely kid to grow up with."
Mayor of London Boris Johnson said he was shocked and saddened to hear about the death of union leader Bob Crow.
Paying tribute to the improvements in London Underground made by his political opponent, he said his death leaves a massive gap behind and praised his indefatigable defence of his members views and interests.
Deputy Prime Minister Nick Clegg has hailed Bob Crow as "a fighter and a force" after the union leader died in the early hours of this morning aged 52.
In one of his last television interviews, RMT union leader Bob Crow spoke to ITV News Meridian on Saturday at a gathering of union members in Brighton to mark the 30th anniversary of the Miners' Strike.
Latest ITV News reports
The late Bob Crow took the trade unions back to their roots and left his members better off - and attracted many critics along the way.
The trade union movement is in shock after the sudden death of rail union leader Bob Crow, who passed away early this morning aged 52