MILAN (Reuters) - Italy's government has extended the deadline for Telecom Italia (>> Telecom Italia) to hand over a report detailing the role its top shareholder Vivendi (>> Vivendi) has in managing the company, a source with knowledge of the matter said on Thursday.

The deadline has been extended to Aug. 23, the source said, adding that Rome had set up an investigation into the matter, giving Telecom Italia (TIM) 10 days to reply.

Italy's government is looking into whether Vivendi breached an obligation to notify it of its effective control of a company considered a strategic national asset.

The source said TIM had asked for an extension of this deadline, and it had been granted. A government spokesman was not immediately available for comment.

Vivendi owns 24 percent of TIM and has slowly tightened its grip on the former telecoms monopolist, where it controls two-thirds of the board. In July, Vivendi placed two of its top executives at the helm.

On Monday, the French media giant said it had no "de facto control" over TIM under Italian law. If Vivendi were to acknowledge such control, it would be forced to consolidate TIM's debt pile.

Rome's investigation is designed to establish whether Vivendi actually controls TIM and if there are the preconditions needed for the government to exercise special powers over the former monopoly.

Under exceptional circumstances, Rome could exercise a so-called "golden power" to block a European buyer if there was serious risk to national security and communications networks.

TIM is considered strategic partly because it owns a submarine network that transmits sensitive information between countries in Europe, the Mediterranean and the Americas.

Vivendi, led by French billionaire Vincent Bollore, could also be fined if it is established that it did not inform the government in a timely manner.

(Reporting by Stephen Jewkes; editing by Steve Scherer)

Stocks treated in this article : Vivendi, Telecom Italia