The International Rugby Board will review the eligibility of Steven Shingler in the wake of the dispute that has broken out over his inclusion in Scotland's squad for the Six Nations.
The Scotland coach, Andy Robinson, named him on Thursday in his 36-man panel for their tournament opener against England at Murrayfield, believing Shingler to be available for selection through his Dumfries-born mother.
The London Irish centre or fly-half, 20, was born in Swansea and the Welsh Rugby Union has contended that he is not eligible to appear for another nation after playing full-back for Wales Under-20s against their French counterparts last year. That appearance would seem to tie him to Wales under IRB Regulation 8.
An IRB statement said: "The International Rugby Board can confirm that it has been contacted by the Scottish Rugby Union and Welsh Rugby Union regarding the eligibility status of Steven Shingler. It would be inappropriate to make any further comment until the matter has been fully reviewed and the unions informed of the outcome."
Meanwhile, James Hook's involvement in the Six Nations has been cast into doubt after it emerged he may not be released by his club, Perpignan, for Wales' training camp in Poland. Their head coach, Warren Gatland, has warned his overseas-based players that they must attend the boot camp to be considered for the Championship, but it clashes with Perpignan's match against Brive in France's Top 14.
Hook's Wales team-mates Mike Phillips and Lee Byrne also ply their trade in the Top 14, with Bayonne and Clermont Auvergne respectively, and have been given permission to attend some of the camp. But Hook remains uncertain whether he will be granted the same leave.
He said: "Perpignan have a big game against Brive the week before the first Six Nations game so it is up to the club. Ultimately, I'm out here and have a job to do.
"It's difficult but all I can do is express my ambitions to play for Wales and if they want me I'm available. Hopefully, I'll be available two weeks before but definitely one week before.
"If the club want me here I'll have to play and I'll put 100 per cent into that. I love the club I'm with now and don't want to upset the applecart here."
Wales' "participation agreement" with the four Welsh regions ensures internationals based at home are released 13 days before Test matches. But there is no agreement with clubs in other countries.
Wales kick off their Six Nations campaign against Ireland in Dublin on 5 February, before welcoming Scotland to the Millennium Stadium a week later.
The Cardiff Blues player Leigh Halfpenny is aiming to land another psychological blow on Ireland before a ball has been kicked. Wales beat Declan Kidney's side in the World Cup quarter-final, and five of the Welshmen who played in that game will be on duty for the Blues against Heineken Cup champions Leinster at the Cardiff City Stadium tonight.
Halfpenny said: "It will give us a massive psychological edge for the game out in Ireland if we can beat Leinster. It would be great if we could go into the game knowing we have had one up on them. The Six Nations is just around the corner, but it's equally important for us at the Blues that we win on Saturday. It's comparable with an international match, without any shadow of doubt. Leinster aren't European champions for nothing."
Cardiff have triumphed only once in their last nine encounters with Leinster but that came in last season's corresponding fixture in Wales.
Halfpenny admitted that if his region are to be considered as serious title contenders in the Pro 12, they must beat the leaders.
He said: "If we want to finish in the top four and make the play-offs we have got to beat the best teams in the league.
"We know we are going to have to be at the top of our game against Leinster. They're a strong outfit, a very physical and well-drilled side who know where they want to go.
"Leinster have got some excellent players and are very dangerous with ball in hand. They've got good finishers and playmakers. It should be an exciting game."
The Welsh region have made four changes to the side that was edged out by the Ospreys on New Year's Day, with Sam Warburton and Gethin Jenkins returning from injury and illness. Gavin Henson remains at full-back.