British Gas and Powergen have already announced price rises

The company said gas bills would rise by an average 5.2% from 1 February, and electricity prices would rise by around 5.8% from 1 April.

Npower said the increases would add around �17.68 a year to the average gas bill and just over �15 to a typical electricity account.

As many as 12.5m British Gas customers will see their costs rise on Saturday following an earlier announcement.

Green energy

Npower said costs for small business customers would also go up at the same time.

Npower calculation Figures are based on an average annual residential usage of customers on standard domestic tariffs of 3,300 kWh for electricity and 20,500 kWh for gas

Npower has around six million customers across the UK, particularly in Yorkshire and the North East.

The company blamed increasing wholesale energy market costs, which have gone up year-on-year by more than 20%, along with an expected rise in gas transportation costs by 5% in 2004.

Npower also said its statutory obligation of sourcing a greater amount of its energy from renewable sources and putting aside money for energy efficiency programmes were also adding to its overheads.

Renewables obligation The percentage of energy that must be renewable rises every year and will increase from 4.3% to 4.9% in 2004, and will increase to 15.4% by 2015

Andrew Duff, Chief Executive of RWE Innogy, npower's parent company, said customers could still make savings, despite the increase.

"Millions of our customers can still cut their bills by paying for gas and electricity by monthly direct debit."

British Gas

Npower is the latest energy company to put up prices.

Powergen's decision in November to put up the costs of energy kicked-in for the majority of its domestic and small business customers on 5 January.

Meanwhile, 12.5m British Gas customers will see their bills rise from Saturday, adding �14.56 a year to an average electricity bill, and �20.28 a year to the bill of an average domestic customer.