In a statement on Monday Eni said it would keep operational control of the unit and would continue to consolidate it in its accounts.

Eni gave no financial details but a source familiar with the matter said the deal valued the stake at around 550 million euros ($601.70 million).

Sources told Reuters in January the deal could value the business at up to 1.2 billion euros.

"The deal is part of Eni's strategy to enhance our assets and free up new resources for the energy transition," Eni Chief Financial Officer Francesco Gattei said.

Sixth Street, which has more than $60 billion of assets under management, operates in infrastructure and energy transmission and has invested in more than 6 gigawatts of renewable power.

Enipower, Italy's second-biggest power producer, runs six gas plants with a total installed capacity of around five gigawatts.

Eni was advised by JP Morgan and Sixth Street was advised by Rothschild.

($1 = 0.9141 euros)

(Reporting by Stephen Jewkes, Editing by Louise Heavens and Susan Fenton)