General secretary of the Rail Maritime and Transport union Bob Crow has died aged 52 from a suspected heart attack.
In a statement on its website, the union said he died in the early hours of Tuesday morning.
The union’s offices were closed for the day on Tuesday.
Mr Crow was elected general secretary of the RMT in 2002 following the death of former leader Jimmy Knapp.
The BBC’s political editor Nick Robinson said Mr Crow "was, some argue, the most successful union leader in terms of securing jobs and pay for his members".
