Sport says PSG president Nasser Al-Khelaifi is angry after learning of Barca moving for the midfielder as the Parisians attempt to open new contract talks with the player's camp.
PSG believe Barca are out of order with their behaviour and have contacted the club's front office to lodge their complaint - and threat.
While FIFA are yet to be contacted, PSG have made it clear they will pursue action if the situation continues.
In response, Barca are furious. The Blaugrana have insisted they've kept PSG in the loop over their plans for Rabiot and have done nothing behind their back.