MILAN (Reuters) - Vivendi’s appeal against an investigation by Italy’s communications watchdog into its holdings in Telecom Italia and broadcaster Mediaset will be heard in court on December 6, three legal sources said on Wednesday. Slideshow ( 2 images ) The watchdog AGCOM began the inquiry into the French media group’s stakes in Mediaset and Telecom Italia in December last year. Aimed at assessing whether Vivendi’s holdings are in breach of national rules granting media plurality, the investigation could help Mediaset, controlled by the family of former Italian Prime Minister Silvio Berlusconi, in its battle against Vivendi, its second largest shareholder. Last month, Vivendi appealed against the regulator’s move, asking an Italian administrative court to annul the decision to start the investigation, which could last up to six months. On Wednesday the court said it would discuss Vivendi’s request on December 6, the sources said. Vivendi and Mediaset were not immediately available for comment. Mediaset and Vivendi have been at loggerheads since 2016 when the French group ditched an accord to buy Mediaset’s pay-TV business and built a 29% stake in the company, a move the Italian broadcaster considers hostile. A legal case has been ongoing ever since. Last year, Rome passed a law giving AGCOM the power to look into Vivendi’s stakes in the country’s media and telecoms sectors and whether, on the grounds of total revenue, entry barriers and the level of competition in those industries, they respect Italy’s rules on concentration of power. Berlusconi, who is at the head of centre-right party Forza Italia, last month joined a national unity government led by former European Central Bank chief Mario Draghi.