Shingler was included in Scotland's provisional squad for the Six Nations, sparking an investigation into his eligibility.
The Scottish Rugby Union believed the 20-year-old qualified through his Dumfries-born mother.
However, the Welsh Rugby Union contended the London Irish star was tied to them having played for their Under-20s side against France in 2011.
And now the IRB have ruled Shingler is eligible only for Wales.
An initial informal assessment by the IRB determined that the player appeared to be tied to Wales and the SRU formally requested the IRB regulations committee to review the case.
A three-man regulations committee panel chaired by Peter Boyle (Ireland) and also comprising Giancarlo Dondi (Italy) and Tim Gresson (New Zealand) heard evidence from Shingler, his agent and the Scottish and Welsh Rugby Unions at a hearing in Dublin on February 27 before announcing their verdict today.
A statement from the IRB read: "The IRB regulations committee panel hearing the Steven Shingler eligibility case has determined that the player is tied to Wales in accordance with IRB regulation eight as it stands and therefore is ineligible to represent another Union.
"The panel determined that, based on the relevant provisions of IRB regulation eight, the relevant guidelines and the facts presented at the hearing, Shingler is captured for Wales in accordance with IRB regulation eight.
"This decision is binding on the parties until it is endorsed or overturned by the IRB council.
"The panel noted that, in accordance with IRB regulation eight, playing in a designated capturing match was the key determining factor with regard to the player's eligibility.
"In this respect the panel accepted the evidence of the Welsh Rugby Union that although Shingler had not signed the Union eligibility confirmation form he had been fully advised by the WRU that playing in the France Under-20 v Wales Under-20 match during the 2011 Under-20 Six Nations would capture him for Wales."
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