Levante have vowed to challenge Lassad Nouioui's move to Celtic, with the Spanish side accusing the striker of breaking an agreement with them.
The Tunisian forward has penned a two-year contract at Parkhead, but now faces the prospect of a switch to Scotland being dragged through the courts.
Levante, who sent Motherwell tumbling out of Europe in midweek, claim Nouioui had already signed for them prior to travelling to Glasgow.
Alberto Gill, a director at the Primera Liga outfit, insists his club will not be letting the issue lie and has revealed that legal advice will be sought.
"Our club reserves the right to use legal matters to investigate this transfer," he said.
"And it is possible it could be reported to FIFA.
"We were told by his representatives that he would be in for training, only to later discover he was already on a plane to Scotland."
Nouioui, though, is turning a blind eye to the controversy he has caused and has expressed his delight at securing a move to Celtic.
The 26-year-old told the club's official website: "I'm absolutely delighted.
"Celtic is a huge club with a fantastic support. I know all about the wonderful atmosphere at Celtic Park and can't wait to play there.
"I know Celtic already have a great squad of players and I'm very much looking forward to joining up with my team-mates. I hope to play my part in bringing success to the club.
"And, of course, I'm also looking to compete in the Champions League with such a great club."