Paris - French rugby club Lyon said on Monday they were taking legal action after South African international Franco Mostert moved to Japan despite a two-year deal with the promoted Top 14 side.
"Lyon Rugby notes with horror that Franco Mostert is in Japan and is training with the Ricoh Black Rams while he has committed to two years with our club," the club said on their website.
The arrival of the 25-year-old lock, who has played four times for the Springboks, had been announced by the French club on March 16 along with eleven other recruits.
"Faced with this serious violation of contract, law and basic values of rugby which are that a commitment be respected, the club will take all the necessary measures to see justice is done and today is approaching the competent legal and rugby authorities," the club said.
Contacted, Lyon refused to give details on which authorities they had approached.
Mostert previously played for Super Rugby side the Lions, who finished runners-up to the Hurricanes in the 2016 final.
He made his debut with the Springboks in June this year, in a 32-26 win over Ireland, and played two of six games in the Rugby Championship which finished in October.