The Scottish Rugby Union insists Steven Shingler is eligible to take his place in their Six Nations squad, despite a challenge from its Welsh counterparts.
The London Irish back was one of six uncapped players named in Andy Robinson's 36-man squad.
But the Welsh claim that Shingler is their player after he played in the second-string Six Nations last season.
"Steven confirmed that he declined to sign a declaration he was committed solely to Wales," said the SRU.
"Steven has informed us that he made it clear that he wished to leave open his international eligibility as he was also eligible to represent Scotland and England as well as Wales.
"Scottish Rugby believes we have acted in good faith in this matter and have also taken cognisance of an IRB ruling last year concerning two players who played for Wales under-20 against France under-20 in 2010 who now play for Connacht and are eligible for Ireland.
"We are liaising with the IRB on this matter."
Welsh Rugby Union
“All the U20 players who represented Wales against France at U20 level in 2011 were informed that they would then become Wales qualified players only at senior level”
The SRU was referring to a ruling relating to Matthew Jarvis and James Loxton in which they believe a precedent has have been set allowing Shingler to feature for Scotland.
Neither Wales nor France field an A team and the Welsh Rugby Union have quoted IRB regulation eight on international eligibility to back their case to the International Rugby Board.
"The Wales U20 team is currently the WRU-nominated side for qualification, which means that players who represent Wales at this level in certain games are then solely qualified for Welsh senior honours," the Welsh insisted in a statement.
"All Welsh players who represent Wales at U20 level against nations which also designate their U20 teams as their qualification sides are informed that they will then be registered as liable for Wales selection only at senior level.
"Shingler played for Wales U20's against France in 2011, before which he was specifically informed, as were his fellow squad members, that the match would confirm their sole qualification as potential Wales senior international candidates.
"Shingler was told by the U20 team management that, if he played in the game, he would become a Wales qualified player and therefore ineligible for selection for any other nation."
Shingler, whose mother is from Dumfries, was looking forward to playing for Scotland despite himself being born in Swansea.
"I would be massively proud to play for Scotland," he said. "My upbringing from my mother means I've always known about Scotland.
"I've got my gran, auntie, two uncles and two cousins in Langholm and, other than the past year, I've been up 15 years on the bounce to see them around the time of the Common Riding and I've competed in the Games as well."
Shingler, who can play at fly-half or centre, was selected by Robinson along with five other uncapped players - Edinburgh's Lee Jones, Sale Sharks lock Fraser McKenzie and Glasgow trio Stuart Hogg, Duncan Weir and flanker Robert Harley.
All 28 available players from the Rugby World Cup squad - Chris Paterson and Nathan Hines have recently retired from the international scene - were named again.
However, number eight Johnnie Beattie again misses out, having been dropped for the squad that travelled to New Zealand following his dip in form with Glasgow.
No announcement was made on the captaincy - currently held by Al Kellock - but flankers John Barclay and Kelly Brown and hooker Ross Ford are all vying for the role.
Scotland will be looking for a positive Six Nations campaign following the disappointment of the World Cup, where they were eliminated at the group stage for the first time.
"I am pleased to reflect on the winning displays we have seen from both Edinburgh Rugby and Glasgow Warriors over the last two months and the contribution a number of our uncapped players have made to them," said Robinson.
"I think what's also encouraging is that a number of players who have forced their way into contention in that period and who have not made the squad this time, are really knocking on the door and that competition for places can only raise our standards, so this Scotland squad can be very much a work in progress over the coming months.
"We said after the Rugby World Cup that the challenge the Scotland squad must confront is to get on with the business of winning international rugby matches."
The squad will gather at St Andrews on Monday 23 January, before being trimmed ahead of the Six Nations opener against England.
Full Scotland squad for RBS 6 Nations:
: J Ansbro (London Irish), M Blair (Edinburgh), C Cusiter (Glasgow Warriors), S Danielli (Ulster), N De Luca (Edinburgh), M Evans (Castres), S Hogg, R Jackson (both Glasgow Warriors), L Jones, G Laidlaw (both Edinburgh), R Lamont (Glasgow Warriors), S Lamont (Scarlets), R Lawson (Gloucester), G Morrison (Glasgow Warriors), D Parks (Cardiff Blues), S Shingler (London Irish) and D Weir (Glasgow Warriors).
: J Barclay (Glasgow Warriors), K Brown (Saracens), G Cross, D Denton (both Edinburgh Rugby), A Dickinson (Sale Sharks), R Ford (Edinburgh Rugby), R Gray, D Hall (both Glasgow Warriors), J Hamilton (Gloucester), R Harley (Glasgow Warriors), A Jacobsen (Edinburgh), A Kellock (Glasgow Warriors), S Lawson (Gloucester), M Low (Glasgow Warriors), F McKenzie (Sale Sharks), E Murray (Newcastle Falcons), R Rennie (Edinburgh Rugby), A Strokosch (Gloucester) and R Vernon (Sale Sharks).