Dublin -The International Rugby Board (IRB) announced on Thursday they believed Scotland hopeful Steven Shingler was "tied to Wales and ineligible to play for any other union".
The uncapped 20-year-old centre and flyhalf, who plays his club rugby for English Premiership side London Irish, has been at the centre of an eligibility row between the Celtic nations since being named in Scotland's squad for next month's Six Nations last week.
The Scottish Rugby Union (SRU) believed Shingler could represent Scotland on account of his mother being born in Dumfries.
However, that position was swiftly challenged by the Welsh Rugby Union (WRU), who pointed out that Shingler had played for Wales Under-20s against their French age group counterparts last season and had, therefore, already determined his senior international allegiance.
Welsh officials sought a ruling from the IRB and it appears the global governing body have sided with the WRU.
In a statement, the IRB said: "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.
"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."
However, the IRB offered the SRU a glimmer of hope by adding the matter could be referred to their regulations committee for a binding ruling.
And the SRU insisted they would press for that review, saying: "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."