Maro Itoje is a doubt for the start of England’s Six Nations campaign after scans confirmed he fractured his jaw in Saracens' Aviva Premiership defeat by Harlequins on Sunday.
The 23-year-old forward collided with Harlequins full-back and England team-mate Mike Brown in the 48th minute of Saracens' 20-19 defeat at the Twickenham Stoop, and was taken to hospital for scans.
Itoje will now see a specialist to determine how long he will be sidelined for, but he faces a race against time to prove his fitness ahead of England's Six Nations opener against Italy on February 4.
England winger Anthony Watson was ruled out for two months when he suffered a similar injury in the autumn of 2016.
The news comes as a blow for both England head coach Eddie Jones and Saracens boss Mark McCall, whose side have lost their last five matches in all in competitions.
Saracens host Clermont Auvergne on Sunday in the Champions Cup in the first of back-to-back matches against the French side before travelling to face Leicester Tigers on Christmas Eve in the Premiership.
Itoje had been making his return for Saracens at the Stoop after being away with the England squad during the autumn internationals. The forward played in only two of England's three wins as Jones managed his playing time.
England are due to travel to Portugal for warm-weather training in January.