Spain's second largest electricity firm has made a takeover approach for Scottish Power, the BBC has learned.
Scottish Power sells electricity and gas
Iberdrola - a proponent of creating a single European market in energy - is the bidder, BBC News Business Editor Robert Peston said.
Earlier, Scottish Power confirmed it had received a bid approach but declined to name the potential suitor.
Shares in the UK firm surged 18% to all -time highs on the news, before closing 58.5 pence, or 8.71%, higher at 730p.
The latest increase in its share price means the firm is now valued at just under £10bn.
Last year, Scottish Power rejected a bid approach of 570p a share from German utility E.ON.
Spanish companies have recently been snapping up UK companies.
Ferrovial of Spain bought airports operator BAA in June, while mobile phone firm O2 was taken over by Telefonica and Abbey was snapped up by Spain's Santander.
Scottish Power, which supplies energy to 5.2 million UK customers, said it was not certain an offer would be received, but if one was made it would be considered.
"The board of Scottish Power confirms that it has received an approach which may or may not lead to an offer being made," it said in a statement.
Scottish Power is made up of four divisions - Energy Retail, Energy Wholesale and Energy Networks in the UK and PPM Energy in the US.
It has electricity generation and gas storage facilities in the UK, US and western Canada.
The company is also the UK's biggest generator and developer of wind energy.