Spanish power company Iberdrola yesterday confirmed it is weighing up a bid for Scottish Power.
Britain's fifth largest energy supplier revealed earlier this week that it had received an approach from an unnamed company, but Iberdrola did not indicate whether talks had already taken place.
A statement from the Spanish firm said that it was "currently evaluating different investment alternatives", adding that its board "had not adopted any decision or agreement in connection with strategic investments".
Shares in Scottish Power rose as much as six per cent on the news but closed down 0.5 per cent at 726.5p.
An offer of about 800p per share would hand the firm, which supplies energy to 5.2 million Britons, a value of £12 billion.
Iberdrola may also be interested in Scottish Power's wind farms in the US. The Spanish company has a current stock market worth of about €32 billion (£21.5 billion).
Last year Scottish Power rejected a 570 pence per share offer from continental utility firm E.ON.