British and Irish Lions captain Sam Warburton is set to miss the rest of this season after undergoing a second operation in three months.

The Wales star will sit out the entire 2018 NatWest 6 Nations following knee surgery.

Warburton, who has been recovering from a neck operation he had earlier this term, is expected to be sidelined for between four and six months, the Welsh Rugby Union has announced.

He has not played since captaining the Lions in the third Test against New Zealand at Auckland's Eden Park five months ago.

The hope now will be that Warburton can enjoy a full pre-season's training ahead of the 2018-19 campaign, which begins a year before the World Cup in Japan.

In a statement, the WRU said: "Sam Warburton has undergone surgery to resolve a knee injury.

"With Warburton currently sidelined with a neck injury, the proactive approach was taken with a view of ensuring he returns to action fully fit.

"The recovery is anticipated between four-six months."

And Warburton added: "In close consultation with the WRU and Cardiff Blues (Warburton's regional team), we decided it would be proactive to have surgery now on a knee injury, and I am looking forward to returning to action fully fit."

Sam Warburton during the 2017 Lions campaign.

29-year-old Warburton has played 74 times for Wales and led the Lions on their last two tours - Australia 2013 and then New Zealand.

The former Wales skipper was on course to recover from a neck operation around the Six Nations kick-off in early February, but he now faces a much longer spell on the sidelines.

Warburton has undergone a number of operations during his career, with a catalogue of previous injuries including a dislocated shoulder, torn hamstring, ankle ligament damage and fractured cheekbone, but he has always returned from such blows and maintained world-class form.

Wales tour Argentina in June, but head coach Warren Gatland has already indicated that a number of his big names will be rested for that two-Test trip.

Aaron Shingler and Josh Navidi filled the flanker roles for Wales in their recent autumn Tests against Australia, New Zealand and South Africa, while Gatland should also have the likes of Justin Tipuric and Ross Moriarty in his Six Nations selection mix.

But Warburton has now followed his Lions and Wales colleague - Scarlets centre Jonathan Davies - in missing the competition.

Davies was injured against Australia four weeks ago, and is unlikely to play again this season, while lock Jake Ball is set to be unavailable for at least the early Six Nations action because of a dislocated shoulder.

Wales begin the Six Nations against in-form Scotland on February 3, followed by a Twickenham appointment with title holders England seven days later.

Warburton's last Wales appearance was against Six Nations opponents France nine months ago.