RAIL, Maritime and Transport union leader Bob Crow, who has died aged 52, leaves an "unfillable void", according to the secretary of the Durham Miners' Associaton.
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 had spoken at several Durham Miners' Galas in recent years.
Last year he told the gathering that it was "fantastic" that former Tory Prime Minister Margaret had died and he called on other unions to leave the Labour Party, saying it no longer served the interests of working people.
His presence on the platform in 2009 caused Labour leader Ed Miliband to pull out of the Gala although he spoke at The Big Meeting in 2012.
Dave Hopper, secretary of the Durham Miners' Association, which holds the Gala, said Mr Crow's death at such a young age was a big loss to the trade union movement.
"He is going to be a massive miss. He has been a very good friend to the Durham miners all the time that I have been in office," he said.
"His word was his bond and whatever he said he was going to deliver he did.
"He is going to leave a terrific void , an unfillable void, in the trade union movement.
"He was a very good speaker and was very popular at the Gala."
Former Mayor of London Ken Livingstone said Mr Crow 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," he said.
Tributes also came from the North-East.
Easington MP Grahame Morris said on Twitter: "Very sad news that Bob Crow has died. The working class and organised Labour have lost a great champion. RIP"
Mr Crow 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."
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."
Darlington-based entrepreneur and Dragons Den star, Duncan Bannatyne, also spoke of his shock.
Writing on Twitter he remarked: "WoW! Bob Crow died from heart attack aged 52. That is so young. RIP"
While Northern TUC tweeted: "Bob Crow fought for workers in our region. Totally supported RMT efforts to lift pay for Metro cleaners."