The 20-year-old was a surprise inclusion in Clive Woodward's Six Nations Championship squads against Scotland and Ireland in February after only a handful of club games.

Duncombe appeared twice as a replacement but was not selected for the forthcoming Argentina tour.

He will concentrate on England Sevens duty instead at this weekend's IRB World Sevens in Cardiff, and is likely to be part of their Commonwealth Games squad.

Duncombe joins other young talents such as Chris Bell, Rob Jewell, Scott Bemand and Luke Sherriff in committing himself to Quins, despite their struggles in the Zurich Premiership this season.

"Over the last couple of years, the identification and development of young players at the club has been given a high priority," said Quins chief executive Mark Evans.

"As we seek to build the future on firm foundations, I am delighted that this group of talented young men have agreed to pledge their loyalty and commitment to Harlequins, even though a number of them had alternative offers."