The Toulouse star faces up to 12 weeks out having undergone an MRI scan after going to hospital on Sunday night suffering from a severe migraine.

Number eight Michael Owen replaces Thomas as skipper against Ireland in Dublin next Sunday with Llanelli Scarlets Lee Byrne taking over at full-back.

Thomas could conceivably be sidelined for the rest of this term, also dealing European Cup quarter-finalists Toulouse a major blow.

He had hoped to travel to Dublin with his Wales team-mates but rest has now been advised and he will remain at home.

Gareth Thomas has sustained damage to an artery in his neck, said Wales team doctor, Professor John Williams.

This caused the symptoms that he experienced on Sunday night and resulted in his admission to hospital.

Those symptoms have since resolved. He has undergone detailed investigations and has been reviewed by a consultant neurologist at the University Hospital of Wales.

The damaged blood vessel now needs to be given time to heal before Gareth can resume training and eventually playing.

We are expecting him to make a full recovery, and it is envisaged that he will be able to resume training after 10 to 12 weeks.

Thomas appeared on a BBC Wales television programme on Sunday night and complained of feeling unwell shortly afterwards.

Caretaker coach Scott Johnson added: We need to do what is in the best interests of Gareth Thomas in this situation, listen to the doctors advice and leave him at home to rest.

We will all be thinking of him, but his personal health is of a much higher priority to us all than anything that can happen on a rugby pitch. We wish him a speedy recovery and hope to have him back with us soon.

One other enforced change from the side that beat Scotland 28-18 nine days ago sees leg injury victim Shane Williams replaced on the left wing by Scarlets back Dafydd James, who wins his 44th cap.

Gavin Henson, meanwhile, is on course to make his first Wales appearance for almost a year when Wales face Ireland.

Henson has not played for Wales since the Grand Slam-clinching victory over Ireland in Cardiff last March.

He missed the autumn Tests after undergoing groin surgery and then incurred a lengthy suspension after being cited for elbowing Leicester prop Alex Moreno during the Ospreys Heineken Cup defeat against Tigers in December.

Henson made his first appearance since the Leicester game last Friday night, when the Ospreys defeated Celtic League opponents Borders and he started at full-back.

He is among three changes on the bench, with Henson replacing Nicky Robinson, Ospreys forward Jonathan Thomas taking over from Scarlets lock Adam Jones and full-back Barry Davies taking Byrnes bench role.

Jonathan and Gavin both figured for the Ospreys last weekend, and they acquitted themselves well, said Johnson.

Jonathan offers us versatility, covering both the back and second row from the bench, and Gavin is covering both the fly-half and inside centre position, which is especially important to us in the absence of Alfie (Gareth Thomas) and Ceri Sweeney.

Lee (Byrne) is a natural replacement for Gareth at full-back, and he has performed well already in that position, both in the autumn and when required against England and Scotland this season.

Dafydd James had an exceptional autumn series and has been unlucky not to figure so far during the Six Nations, but the injury to Shane gives him his chance.

He is in top form at the moment and we are lucky to have such an experienced player to call into the team.

Wales, despite a 47-13 drubbing by world champions England in their RBS 6 Nations opener earlier this month, can still retain the title they won in such scintillating fashion last term.

But they have not toppled Ireland at Lansdowne Road since 2000 and defeat on Sunday would effectively destroy their championship chances.

Apart from the enforced changes we have gone for continuity in selection, which is logical when you realise that all the present coaches have been part of previous selection panels, said Johnson.

Tough times breed tough men, and we are taking a tough team to Dublin to play tough rugby.

We are going to look squarely in the face of adversity and show a different side to our characters this weekend.

Weve shown that we can turn on the pretty stuff, now we are going to show our true heart, make Welsh fans proud of us and proud to be Welsh and come out fighting.

Wales team to play Ireland in the RBS 6 Nations Championship at Lansdowne Road, Dublin:

L Byrne (Llanelli Scarlets); M Jones (Llanelli Scarlets), H Luscombe (Newport Gwent Dragons), M Watkins (Llanelli Scarlets), D James (Llanelli Scarlets); S Jones (Clermont Auvergne), D Peel (Llanelli Scarlets); D Jones (Ospreys), R Thomas (Cardiff Blues), A Jones (Ospreys), I Gough (Newport-Gwent Dragons), R Sidoli (Cardiff Blues), C Charvis (Newcastle), M Williams (Cardiff Blues), M Owen (Newport-Gwent Dragons, capt).

Replacements: M Davies (Gloucester), G Jenkins (Cardiff Blues), J Thomas (Ospreys), G Delve (Bath), M Phillips (Cardiff Blues), G Henson (Ospreys), B Davies (Llanelli Scarlets).