England and Saracens lock Maro Itoje suffered a broken jaw in a weekend Premiership clash, his club announced on Tuesday, meaning he faces a race against time to be fit for the Six Nations.
The British and Irish Lions player was injured in a collision with Mike Brown as Saracens, third in the Premiership, lost 20-19 to fierce rivals Harlequins in Sunday's match at The Stoop.
Saracens have confirmed Itoje's injury is a fractured jaw and the 23-year-old will now undergo further testing to determine his recovery period.
England launch their Six Nations title defence against Italy in Rome on February 4, giving Itoje two months to be fit to contest selection for Eddie Jones' side.
"Scans have confirmed that Saracens forward Maro Itoje fractured his jaw in Sunday's round 10 Aviva Premiership clash against Harlequins," said a Saracens statement.
"The 23-year-old England forward left the field in the 48th minute of Saracens' 20-19 defeat at the Twickenham Stoop and was taken to hospital where scans confirmed the fracture.
"Itoje will now see a second specialist later this week to confirm his return to play schedule."
The second row will miss Saracens' upcoming European Champions Cup clashes, with the double reigning champions next facing French side Clermont in back-to-back fixtures.
Itoje was nominated for World Rugby's world player of the year for 2017 after a stunning calendar year that comprised England's Six Nations win, Saracens' second European triumph and the Lions' drawn series with world champions New Zealand.
England opens its Six Nations campaign on February 4 against Italy, kicking off at 3pm local, Monday 1am AEDT.
Six Nations 2018
Saturday February 3
Wales vs Scotland, Millennium Stadium, 2:15pm local, Sunday 1:15am AEDT
France vs Ireland, Stade de France, 4:45pm local, Sunday 2:45am AEDT
Sunday February 4
Italy vs England, Stadio Olimpico, 3pm local, Monday 1am AEDT
Saturday February 10
Ireland vs Italy, Aviva Stadium, 2:15om local, Sunday 1:15am AEDT
England vs Wales, Twickenham, 4:45pm local, Sunday 3:45am AEDT
Sunday February 11
Scotland vs France, Murrayfield Stadium, 3pm local, Monday 2am AEDT
Friday February 23
France vs Italy, Stade de France, 8pm local, Saturday 6am AEDT
Saturday February 24
Ireland vs Wales, Aviva Stadium, 2:15pm local, Sunday 1:15am AEDT
Scotland vs England, Murrayfield Stadium, 4:45pm local, Sunday 3:45am AEDT
Saturday March 10
Ireland vs Scotland, Aviva Stadium, 2:15pm local, Sunday 1:15am AEDT
France vs England, Stade de France, 4:45pm local, Sunday 3:45am AEDT
Sunday March 11
Wales vs Italy, Millennium Stadium, 3pm local, Monday 2am AEDT
Saturday March 17
Italy vs Scotland, Stadio Olimpico, 12:30pm local, 10:30pm AEDT
England vs Ireland, Twickenham, 2:45pm local, Sunday 1:45am AEDT
Wales vs France, Millennium Stadium, 5pm local, Sunday 4am AEDT