But in a bizarre turn of events Scotland then named the London Irish fly-half in their squad for this year's RBS 6 Nations prompting the Welsh union to ask for clarification from the IRB.
The IRB originally ruled in favour of Wales, only for Scotland to then appeal the decision. However, the organisation debated the issue at their annual meeting in Dublin this month and stuck to their original ruling.
Shingler himself has made no secret of his desire to play for Scotland but now faces an uncertain international future.
An IRB statement said: "Council, having considered all the facts and having heard submissions from the SRU and WRU legal representatives, dismissed the appeal and upheld the decision of the IRB regulations committee.
"The Council therefore determined that Steven Shingler is tied to Wales in accordance with IRB Regulation 8 and therefore is ineligible to represent another Union."
The SRU responded saying: "Scottish Rugby is disappointed by the IRB Council's decision to dismiss the appeal concerning the international eligibility of Steven Shingler. We have sought throughout the process to support Steven in his earnest desire to play for Scotland."