Last month Italy approved stop-gap legislation requiring communications watchdog AGCOM to start a probe into Vivendi's Italian assets, which will last up to six months.

The investigation in set to start on Dec.14, the agenda of a board at the regulator showed on Friday.

Vivendi, controlled by billionaire Vincent Bollore, holds a 29% stake in Italy's top commercial broadcaster Mediaset, and is also top investor in former phone monopolist Telecom Italia (TIM) with a 24% holding.

The watchdog will look to see if Vivendi's positions in Italy's telecoms and media sector are harmful for media plurality by looking at total revenues, entry barriers and the level of competition in those sectors.

The move could help Italian broadcaster Mediaset in a long-running dispute with its second-largest shareholder Vivendi, whose presence the group, controlled by the family of former Prime Minister Silvio Berlusconi, considers hostile.

The French company was forced to transfer two-thirds of its voting rights in Mediaset into an arms-length trust due to a 2017 AGCOM decision which found Vivendi's twin stakes in TIM and Mediaset in breach of Italian laws protecting media plurality.

The trust has been prevented from voting at Mediaset shareholder meetings.

The freeze on voting rights stems from Italian rules setting market share thresholds to prevent excessive power concentration in telecoms and media.

But the EU's top court in September ruled that these measures violate the bloc's rules.

Vivendi has asked an administrative court to scrap the restrictions on its voting rights, with a hearing on the case set for Dec. 16.

Legal sources have said existing restrictions could remain in place until the probe is completed.

(Reporting by Elvira Pollina in Milan and Giuseppe Fonte in Rome; editing by Grant McCool)

By Elvira Pollina and Giuseppe Fonte