Steven Shingler's surprise inclusion in Scotland's Six Nations training squad became hugely controversial last night after the Welsh Rugby Union claimed he was already committed to playing for them.

The 20-year-old had tried to walk out on Wales, for whom he had played in the under-20s championship last season, only to find himself deep in the mire as Scottish Rugby Union officials defended their right to select him.

As matters unfolded it was becoming clear that his case is the latest to highlight the problems caused by the absence of a clear International Rugby Board (IRB) policy which allows member countries to decide which of their teams are their second-string sides.

Under IRB rules, if a player represents a country in either a Test match or at the level immediately below that against another country's first or second team then he is "captured" by that country.

However, this row centres on whether or not the French designated their under-20 team as their second side last season. Unlike Scotland and England, who have A teams, Wales list their under-20s as their second team, but the only match that could tie Shingler – who has birth qualifications to play for Scotland, England and Wales – would be that meeting with France.

The French Federation has changed its designation several times in recent years and Herald Sport understands that it is under no obligation to advise the IRB of when it does so.

While the IRB ruled against the Welsh (and in favour of Ireland) last year when they claimed that two players, James Loxton and Matthew Jarvis, had been tied to them as a result of playing in the under-20 international against France in 2010, it is unclear what the position is regarding the match played in 2011. However, even if the French did designate their under-20 side as their second-string team last year the matter may not be closed because there would be scope for Shingler to appeal if he felt he had not been properly advised of the full implications of playing in that match.

In a statement released last night, the SRU claimed Shingler had refused to sign a document presented to him by the WRU asking for his commitment, which itself implies that they were not sure of the situation.

"Scottish Rugby complies with International Rugby Board regulations and guidelines regarding the status of potentially Scotland qualified players," said their spokesman.

"Today we have named Steven Shingler in our squad for the opening two matches of the 2012 RBS 6 Nations Championship.

"Steven's mother was born in Dumfries and although he played for Wales under-20 against France under-20 last season, Steven has confirmed to Scottish Rugby that he declined to sign a declaration from the WRU that he was committed solely to representing Wales at senior level.

"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."

Earlier in the day Andy Robinson, Scotland's head coach, had been utterly confident that the player was entitled to represent his side when naming him in his man training squad and his main aim had been to ensure that the player's motivation was what it needs to be.

"He had to say he wants to play for Scotland," said Robinson.

"That's a key part for us. We've given him time to think about playing for Scotland and he's put his hand up and said he wants to do that and he's fully committed to doing that . . . so I'm delighted that we have this opportunity to bring him into the squad.

"We found out about him through our scouting. We've got to tap into Scottish talent around the world and bring that into the squad. It's great that we've got a player who is very keen to come and play for Scotland. There are others out there and as well as the home-based players – and as you can see I've brought in four of them – we've got to look at talent all over the world because we want to move this Scotland team forward.

"He had to be passionate about playing for Scotland," Robinson went on.

"I'm not going to twist his arm. We haven't pushed him up against a wall . . . this is about him committing to it and committing fully.

"I'm not prepared to motivate anybody to play for Scotland. They've got to have that desire and will and passion to want to do that and that's got to come with being part of the 22 when selected.

"That applies to every single player. I can do the Churchillian speeches if needed but I've selected the wrong players if I have to motivate them to play for Scotland."

For his part, Shingler clearly believes he is entitled to switch allegiance, emphasising his roots when saying: "I would be massively proud to play for Scotland. 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."