Racing 92 are unhappy after South Africa international Johan Goosen announced his retirement from the sport, aged 24.
The French club said on the day of their must-win European Rugby Champions Cup game at Glasgow that fly-half Goosen is returning to South Africa for a change of career.
He was voted the Top 14's player of the year last season after helping Racing to the title.
A statement from president Jacky Lorenzetti, released on the club's official website, read: "Johan Goosen, whose contract was largely re-evaluated less than a year ago, now claims to be able to get rid of it in a fantastic manner.
"He announced today that he had made the unbelievable decision to stop his career as a player and return to South Africa, with an alleged hiring as a commercial director.
"We regret of course that such a young and talented player is mistaken and declares to abandon professional rugby - as it seems obvious that this role of commercial director is incompatible with a high-level sports practice.
"We would like to add that Racing 92 have always largely supported Johan Goosen both morally and financially, even though he has unfortunately demonstrated lately a certain lack of responsibility towards his partners, his coaches and more broadly of all the members of his club and his supporters.
"Hopefully he will quickly realise that respect for a contract, like that of the given word, is the basis of all human and professional relationships."
The statement suggested legal action would follow, concluding: "Of course, the club reserve the right to give such conduct all appropriate judicial follow-up, both with regard to Johan Goosen and those who advise him."
Goosen has won 13 caps in an international career that started in 2012, the most recent of which came in South Africa's 27-13 defeat to Wales in Cardiff last month.