Swansea-born London Irish fly-half or centre Shingler, 20, was on Thursday named as one of six uncapped players in Scotland head coach Andy Robinson's 36-man squad as preparations for the February 4 Calcutta Cup clash with England at Murrayfield began in earnest.
However, a row has erupted over his eligibility and the IRB are set to determine Shingler's fate.
Shingler played at full-back for Wales under-20s against France under-20s in the 2011 Six Nations, when both nations declared the age-group squads as their second XVs, effectively handcuffing the players involved to those nations.
However, Scotland, who believe Shingler qualifies as a result of his Dumfries-born mother, have been assured by the player that he kept his eligibility open by refusing to confirm his future would lie with Wales.
Following the declaration by the SRU, the WRU issued a further statement, effectively dismissing Shingler's claim and reiterating their stance.
The WRU statement last night read: "Steve Shingler played for Wales U20s 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."
The SRU's statement earlier in the evening read: "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 SRU were referring to a ruling relating to Matthew Jarvis and James Loxton in which a precedent may have been set allowing Shingler to feature for Scotland.
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