Spanish club Levante admit they will fight for their right to have Lassad Nouioui join them.
Levante director Alberto Gill claims that the 26-year old Tunisian forward signed with them from Deportivo La Coruna prior to joining Celtic late last week and will challenge the players move to Glasgow.
"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."
