MILAN (Reuters) - Italy's Atlantia said on Monday it would sell a 29.9 percent stake in Spanish telecoms mast group Cellnex to Edizione, which is the Benetton family holding company and also its controlling shareholder.

Atlantia was granted an option to invest in Cellnex under a deal last month to acquire Spanish toll-road operator Abertis, which owns 34 percent of the telecom tower firm.

Cellnex is Europe's No. 1 player in the sector with a portfolio of more than 27,000 towers across the region. Its total market value is 5 billion euros.

Under the deal, conditional on the Abertis sale going through successfully, Atlantia has the right to sell the Cellnex stake to the Benettons at 21.50 euros per share, cum dividend, for a total of 1.489 billion euros (1.3 billion pounds).

Edizione's offer also includes an earn-out clause in the 12 months following the transfer, Atlantia said.

The sale to Edizione, whose chief executive Marco Patuano knows the towers sector well as a former CEO of Telecom Italia, could trigger further consolidation.

Edizione said in a separate statement that the acquisition represented "a long-term strategic investment with significant growth prospects."

Edizione will give Atlantia the right to co-invest in Cellnex by buying up to 6 percent of the masts company within two years of the execution of the deal.

And Atlantia would also have a "right of first offer" and a "pre-emptive right" should Edizione decide to sell directly or indirectly part or all of the stake within seven years.

Atlantia sold its small towers unit TowerCo to Abertis in 2014. The Spanish firm then spun off its combined masts assets the following year and listed them as Cellnex.

(Reporting by Francesca Landini and Stephen Jewkes; Editing by Alexander Smith)

By Francesca Landini and Stephen Jewkes