Paris Saint-Germain defender Mamadou Sakho potentially faces suspension after joining in obscene anti-Marseille chants during his club's Ligue 1 title celebrations on Monday.
Aboard the open-top bus carrying the PSG squad, Sakho, 23, was seen singing along with the crowd lining the Parisian streets as they insulted OM, their club's arch-rivals.
The France international will now appear before the National Ethics Council (CNE) next month - most likely on June 3 - and could be fined and banned for his behaviour.
Sakho, Paris born-and-bred, apologised for the incident on his Facebook page.
"We were on the bus, and the supporters were singing as we passed," he wrote. "Being a Paris kid, I spontaneously took up a refrain with them. In no way did I do that to inspire hatred, but rather in a jokey tone and to be in harmony with the supporters. I admit it was a mistake, and I would very much like to apologise for it."
The CNE has already shown it takes a dim view of such incidents. In 2011, it handed then-Marseille defender Taye Taiwo a one-match ban after he joined in anti-PSG chants following his club's Coupe de la Ligue victory at the Stade de France.
Last season, several Lyon players were fined and given a suspended one-match ban after insulting their club's main rivals, Saint-Etienne, during public celebrations that followed their Coupe de France win.