The company now has more customers in England

The company said the gain was made in the three months to 30 June.

A marketing push, including a tie-up with supermarket chain Sainsbury's, took total customer numbers to four-and-a-half million.

The firm said a total of 750,000 people were added to its books in the last year alone, adding to a "substantial" rise in profits.

The company now has more customers in England than in its Scottish heartland - a factor that recently led it to double generating capacity south of the border with the �317m acquisition of a gas plant in Kent.

With customer growth boosting margins, ScottishPower said operating profits rose to �24m, although higher costs needed to support the growth offset the performance.

More power

Across the group, pre-tax profits for the three month period were �1m lower at �170m as the company's US-based PacifiCorp business was not as strong as last year, when the weather was unusually hot.

Glasgow-based ScottishPower, which has around half its business in the United States, said there were recent signs of improved trading at PacifiCorp.

In the UK, the deal to buy the Damhead Creek station near the Isle of Grain, Kent, provides ScottishPower with an extra 800 megawatts of generating capacity.

As well as serving its increasing number of customers, power from the company's stations is traded in the wholesale market.

Including the investment in Damhead Creek, ScottishPower said it had earmarked �1.5bn for investments in the year to March 2005.

'Cut prices'

Chief executive Ian Russell said: "This year has started well albeit less strongly than the first quarter of last year.

"Recent trading for the group has been in line with our expectations and our outlook for the year remains unchanged."

However, energywatch Scotland, the independent gas and electricity watchdog, called on the firm to celebrate its improved performance by rewarding its loyal Scottish consumer base, who it claims pay �100 more in some cases than customers in England.

John Hanlon, chair of energywatch Scotland, said: "It is simply unacceptable that ScottishPower charges its local loyal customers in Scotland around �434 a year for an average family home's electricity while it charges the same family in Nottingham �328, in Newcastle �343, in Birmingham �351 and in Leeds �353.

"It is time for ScottishPower and others to show that they are not taking consumers in Scotland for a ride when it comes to electricity prices."

A spokesman from ScottishPower responded by saying it was up to customers to make their own choice over power suppliers.

"Energywatch repeats the same tired refrain every time results are announced. It is clearly frustrated that energy customers are making up their own minds rather than following its diktats," he said.