An Italian union said Monday it will file a criminal complaint against carmaker Fiat SpA after the company refused to allow three dismissed employees back to work despite a court ruling ordering their reinstatement.

The escalating row between Fiat and the Fiom labor union threatens to complicate efforts by Fiat CEO Sergio Marchionne to take on entrenched work practices and introduce more flexibility at Italian plants.

Fiat dismissed the three workers at its Melfi plant in southern Italy in July, accusing them of blocking machinery during a strike to prevent non-striking workers from doing their job. But a court ruled the three should be reinstated.

Fiat is appealing against the ruling and Monday told the three employees they could only carry out work related to their union functions but could not resume their regular jobs at the plant.

They have no access to the production line ... we will file a criminal complaint, Fiom official Enzo Masini told Reuters.

Fiom is the same union that opposes a landmark accord approved by other labor groups and allowing more flexible working conditions at another Fiat plant in southern Italy.

(Reporting by Francesca Piscioneri; Editing by David Holmes)