MILAN (Reuters) - Italy's biggest utility Enel (>> Enel S.p.A.) is on track to raise up to 3.2 billion euros (2.56 billion pounds) from the sale of shares in its Spanish subsidiary Endesa (>> Endesa SA), three sources with knowledge of the deal said on Wednesday.

Enel, Europe's most indebted utility, is selling a stake of up to 22 percent in Endesa to help cut its mountain of debt and beef up the Spanish group's market float.

Bookbuilding for the placement is due to end on Thursday, when the final price is expected to be fixed.

The sources said Enel would offer shares to both retail and institutional investors in a price range of 13.5-13.8 euros.

Endesa shares closed down 1.6 percent on Wednesday at 13.93 euros. Enel shares ended down 0.9 percent.

"The book has been more than one-and-a-half times covered," two of the sources said.

Enel declined to comment.

It will sell a minimum 17 percent stake, and could eventually end up selling more than 22 percent as the placement includes an overallotment option under which the joint global coordinators may acquire between 23 million and 30 million Endesa shares.

The option is expected to be exercised at the beginning of next week, and could approximately raise up to a further 400 million euros, if the maximum number of extra shares are sold at the top price.

Banco Santander (>> Banco Santander, S.A.), BBVA (>> Banco Bilbao Vizcaya Argentaria S.A.), Credit Suisse (>> Credit Suisse Group AG), and JP Morgan (>> JPMorgan Chase & Co.) are joint global coordinators on the deal. Mediobanca (>> Mediobanca Group) advised Enel in the deal while Deutsche Bank (>> Deutsche Bank AG) advised Endesa.

Enel's multibillion-euro acquisition of Endesa in 2008 was a transformational deal that turned it into a major European player overnight.

The state-controlled Italian company, headed by Chief Executive Francesco Starace, is looking to sell more than 4 billion euros of assets this year to help curb its debt and maintain its investment-grade credit rating.

Enel has said it could sell its 66 percent stake in Slovak generating company Slovenske Elektrarne and generation and distribution assets in Romania.

But some analysts have expressed concern the group may have problems finalising such deals before the end of the year.

On Wednesday, Czech electricity producer CEZ (>> CEZ as) said it had sent a letter to Enel expressing interest in buying the Slovenske stake.

Earlier this month Enel raised its forecast for net debt at year-end to 39-40 billion euros, up from a previous estimate of 37 billion euros.

(Additional reporting by Paola Arosio, Editing by Susan Thomas and Mark Potter)

By Stephen Jewkes