Spanish firm Acciona is on the cusp of increasing its stake in compatriot utility Endesa to 24.9%, reports have suggested.

According to Spanish newspaper Expansion, Acciona will add to its current 20% shareholding in Endesa within the next couple of days, causing further concern to rival suitor E.ON.

Spanish renewable energies specialist Acciona has proven to be a thorn in German energy heavyweight E.ON’s side over recent months. Since the company first bought shares in compatriot Endesa in September, it has steadily increased its holding to 20%, making it the largest single stakeholder in the Iberian utility and a significant hurdle for E.ON to overcome in its takeover bid.

Acciona is believed to desire a 30% stake in Endesa, as long as Spanish authorities change the rules so that it would not be required to make a bid for the entire company. However, the Spanish firm says that it does not want to own Endesa outright. Instead, its motivation appears to be to prevent the utility from falling into foreign ownership.