The company said that to cover the increase in wholesale gas prices, domestic bills would need to rise 40 per cent, although industry watchers expect the increase in tariffs to be between 10 and 20 per cent.
The warning of higher bills follows indications from the market leader, British Gas, that it is preparing to announce a swingeing increase in prices of as much as 25 per cent.
It came as ScottishPower unveiled a near doubling in third-quarter pre-tax profits to £328.6m, helped by a 25 per cent increase in earnings from its retail and wholesale division to £52m.
Philip Bowman, the new chief executive of ScottishPower, refused to be drawn on when the price increase would be announced or whether the company would wait for British Gas to put up its tariffs first. But he said current wholesale gas prices made the company's domestic energy business unprofitable.
As a result, ScottishPower is now taking on new customers at a rate of only 3,000 to 4,000 a month, compared with 80,000 a month a year ago. In the nine months to the end of last year it added 90,000 customers or 10,000 a month.
Pre-tax profits for the nine-month period fell 12 per cent to £345m but the results were massively distorted by new international accounting rules which Mr Bowman criticised as being "an impediment to comprehension rather than an aid".
ScottishPower said the better guide to its underlying financial performance was the 34 per cent increase it reported in adjusted operating profit to £559m.
Mr Bowman said the $9bn (£5bn) sale of ScottishPower's US subsidiary PacifiCorp to Warren Buffett may be completed before May. He said he had taken no view on whether or not to keep its US windpower and gas storage business, PPM Energy, but he noted that a number of shareholders and analysts believed that the division should be disposed of once the sale of PacifiCorp is complete.
Mr Bowman conceded that the scale of ScottishPower "may fall behind that of some of our European competitors". But he insisted he had not been brought in to sell the company, saying that it had a viable independent future.Reuse content