It move came hours before EC ruled that Spain's attempts to thwart the advances of E.On were unlawful.

Acciona, a construction and renewable energy company, snapped up the stake in Spain's largest power company through Spanish bank Santander, owner of Abbey National. The company said it had a further 3.6 per cent guaranteed and plans to accumulate up to 24.9 percent of Endesa, the maxium level it can own under Spanish stock market rules without launching a formal takeover bid.

Acciona paid €32 a share for Endesa, 26 per cent more than the amount offered by E.ON and far above the earlier bid by Spain’s Gas Natural.

However, E.ON said it would press ahead with its €29bn bid, which it said depends on it acquiring a minimum of 50.01 per cent of Endesa’s shares.

"E.ON reaffirms its strong commitment in pursuing its offer for Endesa and expects to meet all requirements to proceed shortly," the company said today.

The surprise entrance of a new player in the long-running saga spoiled what would otherwise have been a good day for E.ON, Germany’s largest power company. The European Commission censured Spain for having unilaterally imposed a long list of conditions on E.ON to proceed with the purchase, including the need to shed around a third of Endesa’s generating capacity in Spain.

Spain’s new industry minister has hinted the government is ready to back down in its efforts to stymie the deal.

Yesterday the Spanish government insisted it was not behind the latest move, which some initially suspected of being a new play to keep Endesa in Spanish hands.

Economy Minister Pedro Solbes said yesterday that the government did not have any prior notice of Acciona’s intentions. "The government does not support or oppose this," he said.

Meanwhile, Acciona told the Spanish stock market regulator that the deal was "an independent operation, decided exclusively by Acciona in the framework of its growth strategy".

Acciona has built up a large cash pile during Spain’s decade-long construction boom, and has been looking to diversify away from the highly cyclical business. It has ploughed around €1.5 billion into wind farms, becoming Spain’s second-largest renewable energy player.

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