Cardiff City could face a compensation battle after signing 23-year-old Scotland defender Kevin McNaughton, who had rejected a new deal with Aberdeen.
Cardiff say they have chased McNaughton for two years
A Pittodrie spokesman told the Aberdeen Evening Express: "We plan to fight this all the way.
"We have invested heavily to bring Kevin on over a number of years and believe we should be compensated."
The three-times capped full-back signed professional terms with Aberdeen in August 1999 after being a trainee.
Uefa's ruling regarding compensation payments for players under the age of 24 governs only moves within the same country.
But Aberdeen are set to argue that McNaughton's decision to join the Welsh club is still covered by the ruling as Cardiff are within the United Kingdom.
The Bluebirds say they had been chasing the 23-year-old, who can play in either full-back positions or central defence, for two years but waited until he was a free agent as the fee demanded was too large.
Cardiff owner Sam Hammam told his club website: "This should be a clear sign of our intentions this summer, we will have an aggressive recruitment drive.
"We are targeting a top-six finish next season and I will do anything I can to facilitate that."
The news comes as a boost to Cardiff fans on the day that star striker Cameron Jerome agreed a move to Birmingham.