The RMT has ended talks aimed at averting a strike planned for Sunday by London Underground workers.
A 24-hour walk-out now looks set to go ahead across underground networks in the capital from 6pm.
While a second union is still meeting London Underground to discuss a dispute over jobs and ticket office closures, RMT, the biggest rail union, has walked away.
Officials from RMT and the Transport Salaried Staffs Association had met with LU under the chairmanship of the conciliation service Acas for a fourth day of talks.
But the RMT left the meeting without any sign of progress being made.
TSSA general secretary Manuel Cortes remained at Acas, making it clear he was determined to continue trying to break the deadlock.
The trade union movement is in shock after the sudden death of rail union leader Bob Crow, who passed away early this morning aged 52Read the full story ›
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:
I worked with Bob both during my time at London Underground and here at FirstGroup.
He was a tireless and powerful advocate for his members and was dedicated to the railways and public transport.
Our deepest condolences go out to Bob's family, friends and colleagues.
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.
Sad news. Bob Crow was a fighter and a force. Condolences to his family and friends.
The RMT leader was demonised by Fleet Street, often loathed by commuters, but adored by his members.Read the full story ›