Scotland coach Andy Robinson named the 20-year-old 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 of fly-half was born in Swansea and the Welsh Rugby Union have contended that he is not eligible to play for another nation after playing full-back for Wales Under-20s against their French counterparts last year.
That appearance that would appear to tie him to Wales under IRB Regulation 8, as both sides were nominated as their respective unions' second teams.
And with the respective unions at loggerheads the world governing body has confirmed they will look into the matter after being contacted but the WRU and the Scottish Rugby Union.
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."
The SRU say they have been assured by the player that he kept his eligibility open by refusing to sign a declaration that his future would lie with Wales once he played for the Under-20s, although there is no mention in the regulations of the need for a player to sign an agreement to commit himself to a nation.
The SRU yesterday released a statement that said: "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."
The SRU were referring to a ruling relating to former Wales Under-20 internationals Matthew Jarvis and James Loxton, in which a precedent may have been set allowing Shingler to feature for Scotland.
The pair, who joined Connacht last year, are eligible to play for Ireland despite playing at Under-20 level for Wales.
The WRU's response said: "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.
"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 WRU re-iterated their stance as they confirmed they had contacted the IRB.
Their statement said: "The Welsh Rugby Union has today written to the IRB outlining the process followed to inform all players who represented Wales against France at U20 level in 2011 that their future potential international status was aligned only to selection for Wales at senior level.
"The WRU again emphasised the process followed in relation to the selection of Welsh international squads at senior, U20's and Sevens are fully compliant with IRB regulations.
"As such all Welsh players can be assured of their international status once they have represented Wales in any of the teams relevant for Wales qualification."
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