Harlequins shared the news via a statement on their website, calling the player "one of the club’s most promising youngsters."
The 20-year-old had been at the club since the age of 13, graduating through the academy ranks to play three first-team games.
He also represented England at various youth levels and made four appearances at the under-20 World Cup in Italy last year, where England finished runners-up to New Zealand.
“Our thoughts and prayers are with Seb’s family and friends at this difficult time," said Quins director of rugby Conor O'Shea.
"Seb was an incredible talent and had already represented England at under age level as well as making his senior debut for Quins at just 20.
“He will be sorely missed by the whole squad as well as by the wider Club.”
Tributes have started to pour in from around the game paying tribute to a player who had been a member of the Harlequins Academy since the age of 13.