Billy Vunipola is set to miss England's Six Nations campaign after Saracens announced on Sunday that he had suffered a fractured forearm, dealing a huge blow to England coach Eddie Jones' plans.
The English Premiership club confirmed the injury in a short statement on their website, leaving Jones to plan for a likely campaign without the experienced number eight.
"Saracens can confirm that as a result of an injury sustained in Saturday's European Rugby Champions Cup match against Ospreys, Billy Vunipola has fractured his forearm. Vunipola had an X-ray today confirming the injury," the club said.
"Billy is due to see a specialist later this week, when a timeframe for recovery may be defined. But until that time Saracens will make no further comment. Saracens will issue a further statement once Billy has seen a specialist."
Vunipola's layoff comes just after he had made his comeback from a three-month absence with knee trouble, with the 15-15 draw against Ospreys being only his second game since returning.
The injury is a major blow for Jones, who will name his squad on Thursday for the Six Nations opener against Italy in Rome on February 4.
He is already braced for the unavailability of forward James Haskell, who was shown a red card in Wasps' 33-28 Champions Cup defeat to Harlequins on Saturday.
Tournament organisers European Professional Club Rugby will convene a disciplinary hearing for 32-year-old Haskell, which is likely to take place in the next few days. He could face a ban of up to six weeks.
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