Rewrites, adds details on shareholders' support, buyback plan throughout

MADRID, March 14 (Reuters) - Spanish power utility Naturgy NTGY.MC said on Friday it had set the maximum value of a share buyback announced last month at 2.33 billion euros ($2.53 billion) after its four main shareholders pledged to back the plan.

The company plans to resell the bought-back shares on the market to increase its free float and return to the main stock market indexes, thus supporting their market value.

Naturgy confirmed it would pay 26.5 euros per share, as announced in February. It will buy up to 88 million shares, or 9.08% of its total equity.

Shares traded 0.8% up on Friday morning at 25.14 euros.

Shareholders representing around 85% of the capital committed to participate in the plan, the company said, fulfilling a key condition for it to progress.

These are Spanish holding firm Criteria, its largest shareholder with a 26.7% stake; BlackRock BLK.N - which got its stake through the acquisition of investment fund GIP - and private equity firm CVC CVC.AS, each owning more than 20%; and Australian investment fund IFM, with a stake of almost 17%.

Naturgy estimated in February that increasing its free float to 15% from the current 11% would be enough to achieve its goal of returning to the main indexes.

Shareholders will be asked to approve the plan, along with significant board changes, at the next general meeting on March 25.

($1 = 0.9222 euros)

(Reporting by Inti Landauro and Pietro Lombardi, Editing by Mark Potter)

((Inti.Landauro@thomsonreuters.com;))