Rees has started five of England's last six games
England and Wasps flanker Tom Rees will miss at least the majority of the Six Nations because of a knee injury.
Scans have confirmed the 24-year-old suffered a moderate tear of the medial knee ligament during Sunday's 24-18 Premiership win over Harlequins.
Rees will begin a period of extensive rehabilitation at Wasps.
Rees is not expected to return for a minimum of two months, ruling him out of England's matches against Italy, Wales and Ireland.
England face France on 15 March, while their last Six Nations game of 2009 is against Scotland on 21 March.
Rees missed most of last year's Six Nations after suffering a similar knee injury in the opening match against Wales.
Touted as a future England captain, Rees has started five of the last six England matches.
His absence could see England manager Martin Johnson call up powerful London Irish flanker Steffon Armitage, who is arguably the form number seven in the country and has helped the Exiles to the top of the Premiership.
The 23-year-old Armitage, brother of England full-back Delon, was included in the England Saxons squad in July after an impressive breakthrough season with London Irish.
Leicester flanker Lewis Moody is also ready to reclaim his place in the England squad having missed the summer tour and autumn internationals after suffering a ruptured Achilles and undergoing a hip operation.
Johnson is due to name his new 32-man squad for the Six Nations on 14 January.