The National Professional Footballers' Union (UNFP) of France has called on Paris Saint-Germain to end their exclusion of Adrien Rabiot, claiming that the club's treatment is illegal and borders on "harassment".
Rabiot caused a stir at the end of 2018 when he declined to discuss an extension on his PSG contract, which expires in June.
In response the club banished him to train with the reserves, amid reports that the midfielder would soon be signing terms with Barcelona.
Coach Thomas Tuchel reiterated this week that Rabiot would not be considered for match duties, even though an injury to Marco Verratti leaves PSG short of cover in midfield.
But according to the UNFP, the Ligue 1 leaders are acting improperly, as they lack the necessary facilities to allow a group of professional players to train apart from the first team as stipulated in local regulations.
"When there is no place to train, which is PSG's case, the criteria do not apply, it just means another group cannot be formed," the union's legal director, Stephane Burchkalter, explained to Goal.
"In the case of Rabiot, who has declined to extend his contract which ends at the end of the season, it is not necessary to even consider if this will be a temporary situation.
"No exceptions have been requested, the forming of a second group is now allowed and he should be automatically returned [to the first team]."
The union official added that PSG have until Wednesday to welcome Rabiot back into the fold, and that if they continue to leave him out of training the club could be punished.
"That re-incorporation should be carried out within the next 48 hours," Burchkalter stated.
"If not, the club will be exposed to sanctions, because from a legal point of view PSG have no right to make Adrien Rabiot train with the reserves, just like they did not have the right with [Hatem] Ben Arfa.
"If it is not done in 48 hours, I don't want to get ahead of myself but we will be on guard because right now, PSG are trying to buy time and we do not want them to prevail, especially because the end of the transfer window is approaching. The player is in the right condition to train normally and in line with the regulations.
"If the club continues to evade the rules, it could be seen as a discriminatory measure, which could be compared with harassment, and that is wrong, this should be remedied immediately."
Rabiot's last appearance for the PSG first team came all the way back at the start of December, when he played the last seven minutes of the club's 4-1 win over Red Star Belgrade.