Shingler was named in Scotland's 36-man squad for the Six Nations last week, with Andy Robinson believing the uncapped 20-year-old was available for selection through his Dumfries-born mother.
That was despite Shingler having played for Wales Under-20s against their French counterparts last year.
However, the IRB announced they were to review Shingler's eligibility following a complaint from the Welsh Rugby Union (WRU), and on Thursday they indicated that IRB regulation eight works in the WRU's favour.
A statement from the IRB read: "Under IRB regulation 8, a player is tied to a country if he or she has played for the senior national fifteen-a-side national representative team, or the next senior national representative team, or the senior national representative Sevens team against opposition of the same designation.
"The national Under-20 team can be designated as the next senior national team under the regulation.
"It is the right of a union to choose whatever team it wishes to be designated as its next senior national representative team.
"It is also the sole responsibility of a union to ensure that players selected to play for the teams designated with IRB regulation 8 are eligible to do so.
"The IRB has reiterated to both unions that during 2011, both Wales and France officially designated their Under-20 team to be the next senior national team.
"As the IRB has been advised that Shingler represented Wales Under-20 against France Under-20 in 2011, it would therefore indicate that he is tied to Wales and ineligible to play for any other Union."
The SRU and the WRU have the option of asking the IRB regulations committee to consider the matter formally.
The three-man committee is put in place to rule on matters of interpretation and/or implementation of IRB regulations.
And the SRU wasted little time in confirming that they plan to refer the matter to the committee, as well as reiterating that they are backing Shingler.
The SRU statement read: "We remain fully supportive of the player and his sincere desire to represent Scotland and we intend to refer this matter to the IRB's regulations committee in order that it can consider the case formally in accordance with IRB regulation 2."
The WRU, meanwhile, released a statement of their own expressing their approval of the outcome.
It read: "The WRU welcomes the IRB announcement which confirms that proper procedures were followed when informing the 2011 Wales U20 team to play France they would be solely Wales qualified at senior level after taking part in the game.
"It is important that all players informed of their potential senior selection status now or in future are left in no doubt about the authority of the regulations and protocols in place.
"The case of Steven Shingler being named as an Six Nations Scotland squad player could not be left untested because of the impact on all other players."
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