Wales have suffered a major injury blow with the loss of centre Jonathan Davies from their 2015 World Cup campaign.
Davies ruptured the anterior cruciate ligament in his knee playing for Clermont Auvergne against Montpellier.
The Welsh Rugby Union said on Wednesday that 27-year-old Davies will undergo surgery and is likely to be sidelined for six to seven months.
Davies, who has won 48 caps, would have been an integral part of Wales head coach Warren Gatland's World Cup squad.
Davies' midfield partnership with Jamie Roberts has proved to be a bedrock of Wales' successes under Gatland, and his absence from the tournament that starts in September is a major setback.
In a statement released by the WRU, Gatland said: "It's a disappointment for Jonathan and for us, but the advice is for him to have the surgery straight away.
"Obviously, the injury rules him out of the 2015 Rugby World Cup, but our focus is his long-term career, and we will be working closely with him during his recovery looking towards the 2016 RBS 6 Nations."
Clermont said on their official club website that Davies was hurt in the 33rd minute of the Top 14 game against Montpellier last Saturday, and he went off just after half-time. Clermont won the match 29-17.
The French club said that an examination had revealed a torn anterior cruciate ligament in his right knee.
Experienced Scarlets centre Scott Williams would be the obvious replacement for Davies in Gatland's starting line-up.
Gatland is due to announce a World Cup training squad on June 8, with Wales preparing for a campaign that sees them in the same pool as England, Australia, Fiji and Uruguay.
Wales' opening World Cup fixture is against Uruguay in Cardiff on September 20, with the key England and Australia matches both taking place at Twickenham.
Williams apart, Gatland is also likely to look closely at the merits of young centres like Cory Allen and Tyler Morgan as the World Cup countdown gathers pace.
Davies, though, has been one of Wales' most consistent performers in recent seasons, providing a powerful midfield presence in attack and defence.
He made his Wales debut in 2009 and was a pivotal member of teams that won successive Six Nations titles in 2012 and 2013.
That form was rewarded with a place on the Gatland-coached 2013 British and Irish Lions tour to Australia, with Davies starting all three Tests as the Lions claimed a 2-1 series triumph.
Davies left the Scarlets to join Clermont last year, helping them reach this season's European Champions Cup final at Twickenham, where they lost to Toulon, while his club are also currently involved in the French Top 14 title play-offs.