Wales star Taulupe Faletau looks set to miss most of this season's NatWest 6 Nations after suffering a knee injury.
The powerful number eight, who has won 70 caps and featured on the last two British and Irish Lions tours, was hurt during Bath's European Champions Cup defeat against Toulon last Saturday.
And Bath have now confirmed that Faletau suffered a grade three medial knee ligament injury, which is likely to see him sidelined for 12-16 weeks.
It is understood that Faletau will see a specialist later this week to discover if surgery is required.
"We are really disappointed for Taulupe," Bath rugby director Todd Blackadder said, in a statement released by the club.
"He's been in great form this season, so it's really unfortunate news.
"However, he is in great hands with the medical team, and we look forward to having him back on the pitch soon."
A 12-week absence would rule him out of Wales' first three Six Nations games against Scotland, England and Ireland.
They will visit Dublin on Saturday February 24.
But the whole tournament could prove off limits for Faletau with Wales also facing Italy on March 11 and then France six days later.
Blackadder told reporters on Tuesday: "It's a serious one. We are looking at anywhere between 12 and 16 weeks.
"It's obviously a bit of a blow. I spoke to him after four big (autumn) games for Wales about how he was feeling both physically and mentally, and he was really good.
"It was just one of those things. A guy, sort of off the ball, dives sideways through his knee. These things happen."
Wales are already without two of Faletau's fellow Lions for this season's entire Six Nations, with centre Jonathan Davies and flanker Sam Warburton both sidelined.
And lock Jake Ball is recovering from a dislocated shoulder, meaning he has little chance of featuring during the early Six Nations action.
Faletau suffered two knee injuries last term, and his latest problem surfaced early in the second half of a 24-20 pool loss in Toulon.
A long-term absence would also impact severely on Bath, who remain firmly in the Champions Cup quarter-final hunt and have a number of Aviva Premiership games on the horizon, including a home clash against Wasps.
For Wales head coach Warren Gatland, Faletau's unavailability means he has a considerable void to fill in the back-row.
Gloucester forward Ross Moriarty, who would be a stand-out candidate to replace Faletau, has only played one game this season due to injury, and Gatland could decide to switch the versatile Josh Navidi, who started at openside flanker for three of Wales' recent autumn Tests.
Wales kick off their Six Nations campaign against Scotland in Cardiff on February 3, followed by a trip to face England at Twickenham seven days later.
