In a move that will add to the irritation of German utility E.ON, Spanish spoiler outfit Acciona has increased its stake in compatriot energy firm Endesa by a further 1% as it looks to block E.ON's takeover plan, the Financial Times has reported.

Construction and energy company Acciona now holds 21% of Endesa, taking it closer to a critical mass of shares needed to thwart E.ON’s hopes to acquire its Spanish rival.

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 Endesa in September, it has steadily increased its holding, 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 24.9% stake in Endesa but does not want to own the power firm outright. Instead, its motivation is to prevent the utility from falling into foreign ownership.

The actions certainly have irritated E.ON. The German power giant has filed lawsuits against Acciona in the US and has complained to the Spanish stock market regulator with allegations of collusion.