The uncapped 20-year-old London Irish centre or fly-half was on Thursday named in Andy Robinson's 36-man squad for their opening fixtures against England and Wales, with the coach believing Shingler was available for selection through his Dumfries-born mother despite having played for Wales Under-20s against their French counterparts last year.
That outing would appear to tie him to Wales under the terms of IRB Regulation Eight, as both teams were nominated as their respective unions' second senior sides, meaning players involved in that fixture are effectively bound to their nations.
But Scotland say Shingler had assured them that he had kept his eligibility open by refusing to sign a declaration to commit himself to Wales.
With the unions at loggerheads the world governing body will step in to look into the matter after being contacted by the Welsh Rugby Union 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 WRU reiterated their stance as they confirmed they had written to the IRB over the issue.
Their statement said: "The Welsh Rugby Union have 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, U20s 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."
On Thursday the SRU had said they believed Shingler's refusal to sign a declaration meant he was available to be selected, although there is no mention in the IRB regulations of the need for a player to sign an agreement to commit himself to a nation - the act of playing being deemed sufficient.
They also cited the examples of James Loxton and Matthew Jarvis. The pair played for Wales Under-20s against France in 2010 but following their moves to Connacht last year it was ruled that they were eligible for Ireland.
But it is understood that the WRU believes there is a difference between the two cases in that the 2010 fixture against France, unlike the 2011 game, was not sanctioned as a fixture between two teams nominated for international qualification, as France's second senior side at that time was their 'A' side.
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