Cardiff - Cardiff Blues and Wales hooker Matthew Rees is to undergo testicular surgery, his club announced on Tuesday.
Rees, 32, will now have "an enforced spell away from rugby", a Cardiff statement said.
Former Wales captain Rees, the current Blues skipper, has won 58 caps for his country and played in the 2009 British and Irish Lions Test series against South Africa.
"Cardiff Blues have today (Tuesday) announced that club captain Matthew Rees is to have an enforced spell away from rugby," a Cardiff statement said.
"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 led Cardiff to their shock 19-15 European Cup win over reigning champions Toulon last weekend and would have been expected to be included in the Wales squad for next month's Test at home to South Africa, Argentina, Tonga and Australia due to be announced later Tuesday.
He replaced an injured Ryan Jones as Wales captain against Australia in November 2010, and led his country during the subsequent Six Nations Championship that season.
However, Rees missed the 2001 World Cup in New Zealand due to a neck injury and his current Blues colleague Sam Warburton took over as Wales captain.
Although Richard Hibbard is the current first-choice Wales hooker, Rees remained a member of coach Warren Gatland's squad.