Npower and ScottishPower today became the last of Britain's big six energy suppliers to raise gas and electricity bills, adding at least another £162 a year to millions of customers’ heating costs.

Npower, the UK’s fourth-biggest energy group, will increase gas prices by 26 per cent and electricity bills by 14 per cent.

From today, the company’s 6.6 million customers will pay an average £162 more each year for gas and an extra £60 for electricity.

At the same time, ScottishPower has raised its gas bills by 34 per cent, the second largest rise after British Gas increased its gas prices by 35 per cent last month.

ScottishPower also announced today that it has increased its electricity prices by 9 per cent, which will come into effect on September 1.

The company's 1.2 million customers will pay £221 more a year on gas and an extra £38 for electricity.

Like ScottishPower, npower is blaming today's price rises on “massive” increases in wholesale costs, which it claims has made its previous pricing levels unsustainable.

The company said prices had risen by 122 per cent for gas and coal and by 79 per cent for oil over the last 12 months.

The company, which increased prices in January, claimed that, until today, its domestic gas prices had been the same as they were 18 months ago owing to a decrease in bills in 2007, despite wholesale tariffs more than doubling.

Npower said its electricity generation had also been affected by increases in the cost of raw materials.

Giuseppe Di Vita, managing director of Npower, said: “I’m sorry we’ve had to increase our prices, and we’ve made this decision extremely reluctantly, especially as household budgets are being squeezed so much at the moment.

“There is help available for people who can’t afford to pay their bills and we want our customers to get in touch if they’re worried.”

Willie MacDiarmid, director of energy retail of ScottishPower, said: “These are difficult times and we understand the financial impact this announcement will have on our customers.

"Although we’re one of the last companies to announce increases, we’re sorry we couldn’t hold on any longer. However, we have worked very hard to protect people for as long as possible from these considerable increases in the wholesale market."

However, Gordon Lishman, director general of Age Concern, said: “Many pensioners, already worrying about whether they can afford to heat their homes this winter, will be outraged by news of yet more colossal price hikes.

"It is a huge worry that one in three pensioner households are likely to be living in fuel poverty by the end of 2008 and many are already feeling forced to cut back on essential food or fuel."

Further rises in domestic energy bills will put more pressure on the Bank of England to keep a lid on inflation, which is at a 16-year high of 4.4 per cent. The Bank is due to announce its interest rate decision next Thursday.

Earlier this month, E.ON, which has 5.5 million customers in the UK, said that it would increase prices by 16 per cent for electricity and 26 per cent for gas.

Within hours of E.ON's announcement, SSE, the country’s second-biggest energy supplier, with 8.5 million customers, said that it was raising electricity and gas prices by 19 per cent and 29 per cent respectively

EDF said in July that it would increase electricity prices by 17 per cent and gas bills by 22 per cent.

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