High wholesale prices have forced Scottish Power to slow the rate of new energy customers it attracts.
The announcement came as the group, which also has an infrastructure division dealing with transmission and distribution activities, reported a six per cent drop in first quarter pretax profit after the adoption of new international accounting standards.
The group, which achieves more than half of its business outside its Scottish heartland, said customer numbers increased by 52,000 to 5.16 million in the first quarter to June 30 - down on the growth of 865,000 in the previous year.
It said the division was now focused on gaining "profitable" customers as it was costing more to generate power for households.
The company increased its prices in the spring, but has yet to follow the lead of rivals EDF Energy and Powergen, which announced increases to their tariffs last month. It did not elaborate today on its strategy for household prices.
Last September Scottish Power raised gas prices by 11.8 per cent and electricity prices by eight per cent.
The substantial growth in customer numbers over the previous financial year helped profits at the company's UK division to increase by £47 million to £56 million - although this was also due to the addition of new power plants at Damhead Creek and Brighton.
Scottish Power's infrastructure division also grew operating profit by £22 million to £123 million.
The rise came after watchdog the Office of Gas and Electricity Markets let the unit, which owns various electricity transmission networks as well as managing and maintaining power networks on behalf of other power groups, raise prices.
The UK's fifth largest energy supplier also said it had secured sufficient gas to meet its needs on long-term contracts.
This will enable it to hold prices down by more than its rival Centrica in the face of soaring energy prices.
However, group profit before tax from continuing operations fell to £114 million from £121 million the prior year on revenue up 13 per cent to £1.08 billion.
Net finance costs rose by £25 million to £49 million as a move to International Financial Reporting Standards saw the company take an £18 million charge against its own convertible debt after a rise in its share price.
Pretax profit from continuing operations, stripping out the change to IFRS, rose 40 per cent to £147 million.
The first-quarter figures strip out PacifiCorp, the US-based energy producer which has proved to be a drag on the company's profits.
Scottish Power said it hoped to complete the £5.24 billion sale of the business to US energy giant Mid American between May and November 2006.
The deal will leave the company with its base of UK energy customers, a string of power plants, the distribution arm and US-based PPM Energy - which focuses on clean power sources.