Duncombe, who had made his England debut at the age of only 20, was taken into hospital during a mid-season training break with his club, England coach Clive Woodward mourning the loss of, "one of our brightest and most talented players."
Duncombe's natural pace and attacking flair made him a natural at the Sevens game as well as 15-a-side, and he was a key member of England's ascent up the world Sevens rankings last year, starring for them in the Manchester Commonwealth Games as well as a number of IRB events.
A statement from his club said: "It is with profound regret that NEC Harlequins have to announce that Nick Duncombe passed away yesterday [Friday Feb 14] afternoon, after having been taken into hospital during a week's training in Lanzarote.
"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."
England coach Woodward added: "Nick was hugely popular with the squad and at his club NEC 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.
"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."
England Sevens coach Joe Lydon added his sympathies to Duncombe's family and paid his own tribute on behalf of the England Sevens squad, saying: "All of our thoughts are with Nick's family, friends and team-mates.
"Nick was an outstanding player and one of the most talented and charismatic scrum-halves I have ever seen or worked with. He made a massive contribution to our success in Sevens last year and he was a popular member of the squad.
"It is a tragedy that what I'm sure would have been a long and successful life on and off the field has been cut short in such tragic circumstances."
Planet Rugby would like to send its condolences and wish Duncombe's family and friends all the best at this difficult time.
By Mark Smith