Napoli's decision to fine players for not attending an in-season training camp is being contested by world footballers' union FIFPro.
The training camp was called for by the Serie A club's president Aurelio De Laurentiis following a 1-1 draw against Red Bull Salzburg in the Champions League in November.
Sky in Italy reported last week that some players had received a letter telling them of their sanction in the hours before what turned out to be another 1-1 Champions League draw, against Liverpool at Anfield.
De Laurentiis announced his intention to fine all players who had been due to attend the camp 25 per cent of their monthly wage, with two players - captain Lorenzo Insigne and midfielder Allan - fined 50 per cent.
"Napoli's intention to withhold player salaries is without valid grounds and is being contested collectively," FIFPro said in a statement.
"FIFPRO believes that the ad-hoc decision of Napoli forbidding players to go home and rejoin their families was unjustified.
"Like every other employee, professional football players have the right to ensure that their private life is respected, especially considering the already congested fixture list which sees many players away from home for prolonged periods."
Napoli's players, in particular Insigne, and coach Carlo Ancelotti faced the wrath of supporters at an open training session just after initial story of the fines broke.
Their poor run of domestic form continued on Sunday when they fell to a 2-1 home defeat by Bologna and, although they remain seventh in the table, they are now 17 points behind leaders Inter Milan and five points outside the European places.