Endesa quit neighbouring Portugal's fledgling open market in mid-2007 due to the slow pace of deregulation and price caps on power sales, but changed its stance last December after the government hiked power tariffs to allow better compensation for distributors.

Before it left, Endesa accounted for 8 percent of power sales in Portugal.

"We are back, and we think that this time there will be a true liberalisation. Our goal is to have a market share of around 10 percent in 2010," Javier Uriarte, Endesa general manager in charge of power marketing.

Nuno Ribeiro da Silva, head of Endesa Portugal, said the company already had sales contracts with big industrial clients in Portugal for 1,000 gigawatt-hours of electricity.

Endesa also has a pipeline of generation projects in Portugal worth 2 billion euros, which should start coming onstream in late 2010. 

Power sales in Portugal rose 1 percent last year to 50,583 GWh.

The single Iberian energy market, Mibel, has so far failed to take off because a very low level of electricity in Portugal is currently traded freely and the level of interconnection between Spain and Portugal does not allow for effective liberalisation of the market. (Reporting by Sergio Goncalves, writing by Andrei Khalip)