Duncombe died on February 14 after being admitted to hospital during a week's training in Lanzarote.
The 21-year-old won two caps for England in last season's 6 Nations Championship, making his debut as a replacement against Scotland at Murrayfield.
He was also a key member of England's Sevens squad, appearing in the 2002 Commonwealth Games in Manchester.
Duncombe was one of the brightest young prospects in English rugby, and his untimely passing will cause shock and sadness throughout the game.
Harlequins this morning published a brief statement on the official club website, announcing his death.
"It is with profound regret that NEC Harlequins have to announce that Nick Duncombe passed away yesterday afternoon (February 14), after having been taken into hospital during a week's training in Lanzarote," it read.
"Nick, 21, won two caps for England in last season's 6 Nations Championship, and also represented England in the Commonwealth Games in Manchester last August.
"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."
The news will cast a shadow over the England team and a full house at Twickenham today when England launch their RBS 6 Nations campaign against France.
England head coach Clive Woodward led the tributes to Duncombe.
"The squad were informed of the news that Nick had died suddenly yesterday," 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 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."
England Sevens coach Joe Lydon added: "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."