The International Rugby Board has ruled that Steven Shingler can only play for Wales at international level.
Despite being named in Scotland's Six Nations squad, the 20-year-old played for Wales in an Under-20s match against France in 2011.
The Welsh Rugby Union had alerted the IRB over Shingler's eligibility, believing that his previous appearance meant that he had committed his future to them.
Meanwhile, the Scottish Rugby Union cited two players who had previously played for Wales Under-20s against France Under-20s but later declared for Ireland.
The IRB subsequently claimed that Shingler's Under-20 match held a different status.
"He is tied to Wales and ineligible to play for any other Union," read an IRB statement.
"The SRU and WRU may wish the IRB Regulations Committee to consider this matter formally in accordance with IRB Regulation Two.
"The Unions are aware of the requirements of such a review."
The Six Nations begins on Saturday, February 4.