Paris Saint-Germain will take "appropriate action" against Neymar after he failed to report for training on Monday.
The Ligue 1 champions welcomed players back for the start of pre-season ahead of the 2019-20 campaign, but Brazil star Neymar was absent.
Neymar has been linked with a return to former club Barcelona and now faces a sanction from PSG.
"On Monday July 8, Neymar was due to return to pre-season activities with the Paris Saint-Germain senior squad," said a club statement.
"Paris Saint-Germain notes that Neymar Jr was not in attendance at the agreed time and place. This was without the club's prior authorisation.
"The club regrets this situation and will therefore take appropriate action."
Neymar missed the Copa America with an ankle injury but was expected to be fit for the start of the new season. He was in attendance at the Maracana as Brazil overcame Peru 3-1 in the final on Sunday.
Failing to report for training casts doubt on his future in the French capital, with Real Madrid also having long had a reported interest in the 27-year-old.
PSG president Nasser Al-Khelaifi in June stated "celebrity behaviour" will no longer be tolerated at the club in comments they were widely believed to be aimed at Neymar.
"Players will have to assume their responsibilities even more than before," he told France Football. "It must be completely different.
"They will have to do more, work more. They are not there to please themselves. And if they do not agree, the doors are open. Ciao! I do not want to have any celebrity behaviour anymore."
Neymar has scored 34 Ligue 1 goals in two seasons since leaving Barca for PSG in a world-record €222million move, but injuries have hampered his efforts to help the club win their first Champions League crown.
There have been regular reports he has failed to settle in Paris, with Barca president Josep Maria Bartomeu claiming this month Neymar is seeking an exit from the Parc des Princes.
"We know that Neymar wants to leave PSG, but we also know that PSG don't want him to go, so there is no case for signing him," Bartomeu said.
"I don't speak about other teams' players. We have the maximum respect as an institution. There is nothing we can do."