MADRID, Jan 18 (Reuters) - Iberdrola Chief Executive Ignacio Sanchez Galan appeared before Spain's High Court on Tuesday as part of an investigation into alleged spying in a case that dates back more than 15 years.

Sanchez Galan, who is being investigated for alleged bribery, breach of privacy and fraud in commercial documents, was giving testimony in a hearing behind closed doors.

The court is investigating whether Iberdrola hired former police chief Jose Manuel Villarejo, who ran intelligence services company Cenyt, to spy on Real Madrid soccer club president Florentino Perez when Perez's construction company ACS was fighting for a seat on Iberdrola's board in 2009.

In Spain, being placed under investigation does not necessarily lead to indictment and no formal charges can be brought until the first phase of the investigation is completed.

"We were pleased to have the opportunity to directly address the investigation this week and we continue to reject any allegations of wrongdoing in relation to the contracting of Cenyt. Throughout this investigation Iberdrola has acted with complete transparency," an Iberdrola spokesperson wrote in an emailed response to a request for comment.

Iberdrola, Spain's second-biggest company by market capitalisation, has previously said that Sanchez Galan and three other executives under investigation have done nothing wrong and are open to cooperating with judicial authorities.

The company has conducted three internal investigations without finding evidence of wrongdoing.

Iberdrola has acknowledged reputational damage from its involvement in the case, which started to echo outside Spain in December when regulators in the U.S. state of New Mexico mentioned it in arguments against approval of Iberdrola's bid to buy local utility PNM Resources through its U.S. business Avangrid.

The High Court is also investigating whether Iberdrola hired Villarejo to repel local opposition to a power plant in southern Spain and to obtain information about Manuel Pizarro, former chairman of utility Endesa.

The investigation is part of a wider inquiry, which is centered on Villarejo's activities and has roiled Spain's corporate sector.

Spanish bank BBVA remains under investigation in relation to Villarejo's activities while investigations into Repsol and Caixabank were dropped last year.

(Reporting by Emma Pinedo and Isla Binnie Additional reporting by Nacho Doce Editing by David Goodman)