Keighley Cougars rugby league player Danny Jones has died after suffering a suspected cardiac arrest during a game.
The Wales international, 29, collapsed after being replaced during the League 1 game at London Skolars on Sunday.
The married father of five-month-old twins was treated at the ground before being taken to the Royal Free Hospital.
"He was the life and soul in the dressing room, a natural leader, a true professional and irreplaceable," read a Keighley Cougars statement.
Jones, from Halifax, was injured in the fourth minute of the game at the New River Stadium and was treated by the match doctor and paramedics at the ground.
London Ambulance Service said they were called "to a patient in cardiac arrest" at the ground at 15:10 BST and "extensive efforts were made to resuscitate him."
The contest was abandoned in the 16th minute with Keighley 12-6 ahead.
"It is always devastating when someone so young dies in these circumstances," said Rugby Football League chief operating officer Ralph Rimmer.
"Danny was a popular and talented player, having played at the highest level with Wales and for more than 12 seasons for Keighley Cougars and Halifax."
Jones scored in excess of 1000 points in 150 appearances for Keighley and won 12 caps for Wales, making his debut against Italy in Wrexham in 2010.
His final international appearance was in the 2013 Rugby League World Cup, against Cook Islands in Neath.
|Deaths in rugby league|
|Jones is the second rugby league player in just over six years to die after collapsing during a match. Wakefield forward Leon Walker, who was 20, fell ill during a reserve game with Celtic Crusaders in Maesteg in March 2009. It was later discovered that he had an undiagnosed heart defect and a coroner ruled that he died from natural causes.|
|Just six months earlier, in October 2008, Wakefield's Cook Island international Adam Watene, 31, died after collapsing during training.|
|Leeds half-back Chris Sanderson died on the pitch in a match in 1977.|
Rimmer added: "The rugby league community will mourn his loss and I wish to extend our deepest sympathies to Danny's family and friends at this incredibly sad time.
"The family, friends and team mates of Danny are currently, and will continue to be, supported by the RFL Benevolent Fund and Sporting Chance.
"We would ask that the privacy of the Jones family, players and Keighley Cougars staff be respected."
Tributes to a "great man"
BBC Super League Show presenter Tanya Arnold said: "Such sad news about Danny Jones, thoughts with all those close to him at such a difficult time."
England international Chris Hill also paid tribute to Jones on Twitter. "Such sad news about Danny Jones," he said. "Played against him a few times always a top competitor; thoughts go to all his friends and family."
Wales Rugby League chairman Brian Juliff said: "Everyone at Wales Rugby League is very shocked to hear of Danny's sudden passing. Our condolences go out to his family, friends and colleagues at this sad time."
The sport's players' association, League 13, said in a statement: "Our deepest sympathy and condolences go to the family of Danny Jones tonight."