UPDATE 1-Acciona Energia plans to start trading shares on July 1

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MADRID, June 21 (Reuters) - The renewable energy unit of Spanish engineering group Acciona on Monday said it expected to make its debut on the Madrid stock market on July 1, in what is set to be one of the biggest listings in a record year so far in Europe.

Acciona hopes the listing of up to 25% will value Acciona Energia at as much as 9.8 billion euros ($11.66 billion). The parent company will maintain a controlling stake.

A prospectus for the deal showed it was aiming to price the shares, which will all be existing stock, between 26.73 euros and 29.76 euros each, raising as much as 2.45 billion euros.

A so-called greenshoe over-allotment option of between 10% and 15% of the offer could boost this to 2.817 billion euros.

This is the biggest IPO in Spain in more than six years, and has been one of the most closely watched deals in Europe. It is set to give the energy unit a higher market value than the 7.1 billion euros currently ascribed to its parent.

Order books have been opened, one of the banks acting as bookrunner said.

Renewable energy stocks climbed steadily throughout 2020 but have pulled back from their January highs, casting some doubt over whether the sector would be as warmly viewed by investors as previously.

Smaller renewable firm OPDenergy pulled a planned Madrid IPO earlier this year, while Ecoener downsized its transaction. ($1 = 0.8408 euros) (Reporting by Jesús Aguado; editing by Isla Binnie)