French power group GDF Suez (GSZ.FR) Friday confirmed it sold its entire stake of 5.01% in Spanish energy company Gas Natural SDG SA (GAS.MC) for EUR540 million in cash.

In a brief statement, GDF Suez said that this "sell-down is part of the optimization strategy of the group financial resources."

Earlier, Citigroup Inc. (C) and Goldman Sachs Group Inc. (GS) said they were selling the 46.18 million shares in an accelerated bookbuilding process to institutional investors.

The sale price was EUR11.70 a share, the banks said in a follow-up release Friday.

Following this transaction, GDF Suez will no longer be a shareholder of Gas Natural Fenosa, it also said.

Suez started building a stake in Gas Natural in 2005 through a joint-investment vehicle, Hisusa, with Spanish savings bank La Caixa, increasing its stake at one point to 11% in 2007 amid market opportunities, a company spokesman said. After GDF and Suez combined, the newly merged company began to divest. Today's sale comes as GDF Suez seeks to reduce its debt following its deal with International Power PLC (IPR.LN). GDF Suez has committed to sell EUR4-5 billion of assets.

Suez Environnement SA (SEV.FR), in which GDF Suez has a leading stake, sold down its position in Gas Natural in October last year.

"They got the stake indirectly, through a transaction, and this has never been strategic to them. They kept saying it never was and the stake was not consolidated into their accounts," said a Paris-based analyst, who declined to be named.

"They don't lose money over this, there is nothing spectacular in this transaction," the analyst added.

Gas Natural had no comment.

Gas Natural's market capitalization Thursday was EUR11.19 billion, making a 5% stake worthEUR560.6 million.

Gas Natural shares were suspended early Friday. When they resumed trading, they were down 3.6% at EUR11.70, amid a flat Spanish market.

At 1027 GMT, Gas Natural shares were down 4.1% at EUR11.64, while shares in GDF Suez were trading down 1% to EUR26.66 in
Paris .

Gas Natural shares fell as the per-share sale price was below Gas Natural's share price before the sale, Bankinter analyst David Garcia Moral said.

He added that Gas Natural had also already been weakened by a court ruling in August ordering it to retroactively pay more for natural gas supplied by
Algeria 's Sonatrach. Moral has Gas Natural's target and recommendation under revision.