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.
“RMT would request that all media respect the privacy of the friends and family of Bob Crow at this distressing time.”
52 year old Mr Crow was well known to Londoners having led his members in a series of high profile disputes with Transport for London and Mayors Ken Livingstone and Boris Johnson.
He also served briefly on the board of TfL during Mr Livingstone’s first term at City Hall.
Reacting to news of Mr Crow’s death, Livingstone told ITV: “His members are one of the few groups of working class people who have still got well paid jobs.”
Mayor 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.”
Manuel Cortes, General Secretary of the TSSA union which also represents Tube workers, said: “Devastated to hear of Bob Crow’s untimely death. My thoughts are with his family, friends and the members of our sister union. RIP comrade!”
Labour London Assembly Member Tom Copely said: “Whatever you think of his tactics he was a hugely successful union leader.”
Paul Embery, Regional Secretary of the Fire Brigades Union in London, described Mr Crow as “a true hero of the labour movement, and a great friend of the Fire Brigades Union.”
