French media giant Vivendi on Friday filed a legal challenge against Telecom Italia's plans to sell its fixed-line network to US investment fund KKR, according to a source close to the matter.

Vivendi, the main shareholder in the Italian telecommunications firm with a holding of 23.75 percent, had slammed the deal approved last month as "illegal".

Its appeal to a Milan court aims to cancel the sale but without an emergency suspension of the transaction, meaning the sale process continues, the source told AFP.

Vivendi did not respond to requests for comment.

Telecom Italia (TIM) said in a statement that Vivendi had informed it of the legal challenge, but said the sale to KKR was going ahead as planned.

Telecom Italia is seeking to sell its fixed network to pare down a huge debt pile that stands at more than 26 billion euros ($28 billion).

In early November, its board approved a deal with KKR worth up to 22 billion euros, which it said at the time would reduce the debt by "around 14 billion euros".

Vivendi, which says this significantly undervalues the network, had called for an extraordinary general meeting of shareholders to approve the deal -- where Vivendi would carry considerable weight.

"The rights of Telecom Italia shareholders are being trampled on," the firm said at the time.

The Italian government is the second-largest shareholder in TIM, which was privatised in 1997.

Prime Minister Giorgia Meloni's government intends to take a stake of up to 20 percent in the fixed-line network, viewing it as strategic infrastructure.

If the deal goes through, TIM would become the first major operator in Europe to sell its fixed network on its domestic market to slash debt.

The debt load is hampering TIM's efforts to invest in rolling out fibre optic networks, where Italy is lagging behind other advanced economies.