Scotland and Wales were locked in a Six Nations row after Swansea-born back Steven Shingler was named in the Scottish Six Nations squad, despite having played for Wales at age-group level.
The 20-year-old London Irish flyhalf or centre was named in coach Andy Robinson's 36-man squad ahead of the February 4 Calcutta Cup clash with England at Murrayfield.
But the International Rugby Board will now have to settle the row over his eligibility.
Shingler played at fullback for Wales Under-20 against France Under-20 in 2011 when both nations declared the age-group squads as their second XVs.
"All the U20 players who represented Wales against France at U20 level in 2011 were informed that they would then become Wales-qualified players only at senior level," the Welsh Rugby Union said in a statement.
But Scotland, who believe Shingler qualifies as a result of his Dumfries-born mother, have been assured by the player he kept his eligibility open.
"Steven has confirmed to Scottish Rugby that he declined to sign a declaration from the WRU that he was committed solely to representing Wales at senior level," the SRU responded in a statement.
"Steven has informed us that he made it clear that he wished to leave open his international eligibility as he was also eligible to represent Scotland and England as well as Wales. We are liaising with the IRB on this matter."
The Scottish Rugby Union argues that Shingler was selected in good faith after assuring Scottish selectors that he had deliberately not signed a Welsh Rugby Union statement committing himself to Wales.
The SRU statement: "Scottish Rugby complies with International Rugby Board regulations and guidelines regarding the status of potentially Scotland qualified players.
"Today we have named Steven Shingler in our squad for the opening two matches of the 2012 Six Nations Championship. Steven's mother was born in Dumfries and, although he played for Wales Under-20 against France Under-20 last season, Steven has confirmed to Scottish Rugby that he declined to sign a declaration from the WRU that he was committed solely to representing Wales at senior level.
"Steven has informed us that he made it clear that he wished to leave open his international eligibility as he was also eligible to represent Scotland and England as well as Wales. Scottish Rugby believes we have acted in good faith in this matter and have also taken cognisance of an IRB ruling last year concerning two players who played for Wales Under-20 against France Under-20 in 2010 who now play for Connacht and are eligible for Ireland.
"We are liaising with the IRB on this matter."
The Welsh Rugby Union statement: "The Wales U20 team is currently the WRU nominated side for qualification which means that players who represent Wales at this level in certain games are then solely qualified for Welsh senior honours.
"All Welsh players who represent Wales at U20 level against nations which also designate their U20 teams as their qualification sides are informed that they will then be registered as liable for Wales selection only at senior level.
"Steve Shingler played for Wales U20's against France in 2011 before which he was specifically informed, as were his fellow squad members, that the match would confirm their sole qualification as potential Wales senior international candidates.
"Shingler was told by the U20 team management that if he played in the game he would become a Wales qualified player and therefore ineligible for selection for any other nation.
"The regulations and guidelines relating to qualification compliance are set in accordance to IRB regulations and any questioning of these regulations should be directed to the IRB.
"We have made the IRB aware of this issue today and will be returning to it as a matter of urgency in the morning."
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