Former Wales captain Matthew Rees is to undergo testicular surgery.
The 32-year-old Cardiff Blues hooker faces "an enforced spell away from rugby," the Blues announced in a statement.
Rees has won 58 caps and played in the 2009 British and Irish Lions Test series against South Africa. He is the current Blues captain.
The Blues said: "Club captain Matthew Rees is to have an enforced spell away from rugby.
"Hooker Rees, who joined the region from Scarlets this summer, will miss Friday's league trip to Ulster as he will undergo testicular surgery.
"Cardiff Blues would like to take this opportunity to wish Matthew a speedy recovery, and we look forward to seeing him back in action in the future.
"The region will be making no further comment and would ask that the media respect Matthew's privacy at this time."
Rees would have been expected to be named in the Wales squad for next month's Tests against South Africa, Argentina, Tonga and Australia, which is being announced on Tuesday.
Rees replaced an injured Ryan Jones as Wales captain against Australia in November 2010, and he skippered his country during the RBS 6 Nations Championship later that season.
He missed the 2001 World Cup in New Zealand, though, due to a neck injury and his current Blues colleague Sam Warburton took over as national captain.
Although Richard Hibbard is the current first-choice Wales hooker, Rees had remained an important part of the squad under head coach Warren Gatland.
Rees began his career with Welsh club side Pontypridd before joining the Celtic Warriors until they were dissolved, He then went on to make more than 170 appearances for the Scarlets, and linked up with the Blues earlier this year.
Wales boss Warren Gatland's squad for the November games could also be notable for the absence of James Hook.
The Perpignan star, who has won 70 caps, looks set to miss out, Press Association Sport understands.
Hook has not started for Wales against another Test-playing country for two years, although his form for Perpignan this season has been outstanding.
Elsewhere, it is thought that Gatland will name Racing Metro centre Jamie Roberts in the squad, although he is currently recovering from injury, with Warburton set to continue as captain.
Scrum-half Mike Phillips is also set to feature, despite him being summoned to face directors at his club Bayonne on Tuesday after turning up for a players' video analysis session earlier this month while allegedly under the influence of alcohol.