Wales and Cardiff Blues flanker Sam Warburton is to have surgery on a long-standing neck injury, ruling him out for up to four months.
The 28-year-old, who captained the British and Irish Lions in their drawn Test series with New Zealand this summer, aggravated the injury in training this week.
The operation means Warburton will miss all four of Wales' autumn Tests.
He will also miss key Pro14 and Challenge Cup dates for the Blues.
"It is a major blow for all concerned, for Cardiff Blues, Wales and Sam Warburton," said Blues head coach Danny Wilson.
"It is a long standing issue Sam has had. He gets pain and discomfort on and off.
"The feeling was that the two months off [after the Lions tour] to allow that to completely settle without any rugby would do him the world of good, which to be fair it did.
"He is frustrated because he would have thought after two months he would have been quite fresh. But it is the right thing to do.
"For him personally it's another frustration in terms of an injury he has to come back from."
Warburton missed six weeks at the end of last season with a knee injury, while in October 2016 he suffered a fractured cheekbone that put him out for three weeks.
He has also had several serious shoulder injuries during his career and also missed the deciding Lions Test against Australia in 2013 with a torn hamstring.
Wales have back-row injury concerns ahead of their autumn internationals against Australia, Georgia, New Zealand and South Africa with fellow Lion Ross Moriarty also currently sidelined.
Ellis Jenkins (hamstring) and James Davies (shoulder) could have been possible replacements for Warburton but have also been ruled out of the autumn series.
Ospreys flanker Dan Lydiate has not played following a knee injury suffered playing for Wales in November 2016.
The Blues already have injury problems in the back row, with Jenkins and James Botham (ankle) also ruled out for up to four months.
That leaves Josh Navidi as the only senior specialist open-side flanker.