Spanish power utility Iberdrola vowed to keep up its battle to prevent unwanted shareholder ACS exerting more power in the company despite losing a court case.

In a decision released on Friday, Spain's Supreme Court rejected Iberdrola's challenge to a new law scrapping a company's power to restrict individual shareholder voting rights to 10 percent.

Building group ACS, which owns 18.55 percent of Iberdrola, should now be able to vote in accordance with its full stake.

"This is good news for ACS, although it still doesn't guarantee them a seat on Iberdrola's board, and Iberdrola is clearly keeping up the fight," said an equities analyst for an international brokerage in Spain who asked not to be named.

ACS, headed by Real Madrid soccer club President Florentino Perez, has spent nearly three years trying to get more control over Iberdrola as it tries to diversify away from Spain's ailing construction industry.

Perez has said he wants 30 percent of Iberdrola, but his moves have met with opposition from Iberdrola, which has resisted its stakebuilding attempts and taken the matter to court on various grounds.

Aside from limiting ACS's voting rights, Iberdrola has kept ACS from joining its board, arguing that conflicts of interest between the two companies in areas like energy make it a competitor.

Its claim was recently upheld by a court in Vizcaya, northern Spain.

That may change if ACS continues to sell off its energy assets and can convince the court it is not a direct competitor.

Shares in both companies were little changed by Friday's ruling, which traders said was widely expected. Iberdrola was off 1.7 percent at 3.83 euros and ACS lost 1.9 percent to 17.21 euros, in line with a 2 percent fall on Spain's main index.

Iberdrola said it plans to ask the Supreme Court to annul the ruling, a procedure that would allow the company to appeal to the Constitutional Court of Spain.

"Within the 20-day calendar Iberdrola will present a petition to annul the ruling," a spokesman for the utility said on Friday.

Legal sources said the Supreme Court rules against annulment requests in 99 percent of cases but that the procedure is necessary in order to appeal to the Constitutional Court.

ACS's power to vote at Iberdrola's next shareholders' meeting could tip certain issues to its favour, although so far the utility has enjoyed strong support from its other shareholders.

After ACS the next biggest shareholder is Qatar Holding, the direct investment arm of the Qatar Investment Authority with 8.45 percent of Iberdrola, followed by banking group BFA with 5.35 percent and Basque savings bank BBK with 5.3 percent.

Iberdrola, which is also waiting for the government to reveal a new energy sector policy to address a 24 billion euro power tariff deficit, has yet to announce the date of its annual shareholders' meeting.

The Supreme Court's ruling on Friday will also affect other companies that had limited shareholders' voting rights to 10 percent such as Telefonica and Repsol.