England flanker Tom Croft has seen his RBS 6 Nations campaign ruined after scans confirmed the damage from the knee injury he suffered on Saturday.
Croft had to be helped off after 27 minutes of Leicester's 47-8 win over Viadana in the Heineken Cup and faces a five to six-week lay-off.
The 24-year-old will therefore miss his country's opening two matches against Wales and Italy, and his participation against Ireland at Twickenham on February 27 is also extremely doubtful.
A statement from the Rugby Football Union last night read: "A scan has confirmed that Tom Croft sustained a grade two medial cruciate ligament injury to his left knee in the Heineken Cup game against Viadana last weekend.
"He has now started his rehabilitation programme with a view to returning to action in five to six weeks."
Croft established himself in the international line-up during last year's Six Nations, coming on a substitute in the first three games and earning a starting spot for the final two matches against France and Scotland.
He was handed a late call-up to the British and Irish Lions' summer tour of South Africa after Munster's Alan Quinlan was ruled out by suspension, and scored a try in the first tour match against the Golden Lions before adding two more in the first Test against the Springboks.
He played a key role for the Tigers in the early weeks of this season before an injury to the same knee sidelined him for six weeks - the Viadana game was just his second appearance since returning from that lay-off.
Northampton's Courtney Lawes will be one of the men in contention to replace Croft after being promoted to England manager Martin Johnson's senior squad ahead of the tournament.
Harlequins back-rower Nick Easter is also back in the 32-man party after injury.