French club Paris Saint-Germain said they would take "appropriate action" after Neymar failed to show up for pre-season training with the French champions.
Neymar, the world's most expensive player after moving to PSG for 222 million euros ($252 million) two years ago, had watched his country win the Copa America in Rio de Janeiro on Sunday night.
But when PSG players including France World Cup-winning striker Kylian Mbappe reported back for training and pre-season medical checks in the French capital on Monday, Neymar was nowhere to be seen.
PSG said in a statement: "On Monday 8 July Neymar da Silva Santos Junior was due to return to pre-season activities with the Paris Saint-Germain senior squad.
"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."
Brazilian media quoted Neymar's father as saying that PSG "were aware" the striker would be unable to return to the club for another week.
"The reason was known and planned for a year as part of his activities for the Neymar Institute (charity foundation)," said Neymar Senior.
He added: "We couldn't postpone them and he will return (to PSG) on July 15.
"It's as simple as that, no drama. PSG are aware and take part in the Institute's activities."
Neymar, 27, is undergoing treatment on the torn ankle ligaments that ruled him out of the Copa America.
- 'PSG won't sell' -
His absence is likely to fuel rumours that he could make a surprise move back to Barcelona, the club he left to join PSG, this summer.
Barcelona president Josep Maria Bartomeu downplayed that speculation on Tuesday ahead of the unveiling of new signing Frenkie de Jong at Camp Nou.
Bartomeu said Paris Saint-Germain president Nasser Al-Khelaifi was not prepared to sell Neymar this summer, so a deal was unlikely to happen.
"We know that he wants to leave Paris Saint-Germain but we know that they don't want him to leave, so there is no case," he said.
"I know what I have read in the press. We have contact with Nasser in meetings and committees but he has not told us himself. What we know about Neymar we know from the press."
French media noted that what was striking about Neymar's latest refusal to toe the line was PSG's lack of tolerance of his behaviour.
After the club said it would take action, PSG's newly installed Brazilian sporting director Leonardo told Le Parisien newspaper that "Neymar can leave PSG if there is an offer that suits everybody."
Sports daily L'Equipe's headline was: "With Leo (Leonardo), there's no longer any joking around."
Despite the denials, Spanish sports newspaper Diario Sport reported that intermediaries for the two clubs have negotiated a five-year contract with Neymar accepting a cut in salary from around 36 million euros a year at PSG to the 24 million euros he received at Barca in 2017.
In four years in Catalonia, Neymar won the Champions League in 2015 and two La Liga titles, forming a brilliant attacking trio with Lionel Messi and Luis Suarez.
Any return would likely require Barca to raise funds through sales. De Jong has already signed for 75 million euros from Ajax. Griezmann is expected to join from Atletico Madrid for 120 million euros, although that deal has hit snags.
Neymar's time at PSG has been punctuated by injury and controversy.
He was questioned by Brazilian police last month over allegations he raped a woman he met through social media in a Paris hotel. He has denied the accusations and his father says it was a blackmail attempt.
Neymar will miss three Champions League matches next season as punishment for insulting a referee following PSG's Champions League collapse against Manchester United.
Al-Khelaifi has warned Neymar and his other highly-paid stars that they must shape up or ship out, claiming "celebrity behaviour" will not be tolerated.
"The players will have to assume their responsibilities even more than before," Al-Khelaifi told France Football last month.
Injured Neymar and his son Davi Lucca watch Sunday's Copa America final at Rio de Janeiro's Maracana stadium from the stands
Neymar played a handful of Ligue 1 matches for PSG before the end of the season after returning from injury