The Harlequins number nine was taken to hospital during a week's training in Lanzarote and died on Friday.
A spokesman for Harlequins said: "At this very sad time, everyone at Harlequins would wish to pass on their condolences to Steve and Helen, Nick's parents, and the rest of his family."
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The two caps he gained with England last year would have been the beginning of a long international career
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England boss Clive Woodward
Duncombe would have been in contention for the England squad to face France at Twickenham on Saturday but had earlier been ruled out because of injury.
He had been struggling with a hamstring complaint but had recently returned to training.
As a tribute there will be a minute's silence before the game.
Duncombe was twice capped by England during last season's Six Nations championship and was also part of the national sevens side at last summer's Commonwealth Games.
As a player, he had enjoyed a meteoric rise in the rugby fraternity.
He made his debut for Quins in January last year and was picked for England by Clive Woodward with just 270 minutes of professional rugby under his belt.
Woodward led the tributes to Duncombe.
Nick Duncombe profile
He said: "Our immediate sympathies and thoughts are with Nick and his family at this difficult time.
"Nick was one of our brightest and most talented players in the game.
"The two caps he gained with England last year, would have been, I believe, the beginning of a long international career, which had already started so well with some superb performances for the England under-19 and Sevens teams.
"Nick was hugely popular with the squad and at Harlequins and his attitude to the game was outstanding. I'm sure that all supporters and players will share the loss of one of rugby's brightest stars."
Originally an academy player at Saracens, Duncombe was brought to the Stoop by Quins chief executive Mark Evans.
His early career, however, had been blighted by serious injury when he broke his neck in an under-18s international against Wales in April 2000.