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

This year energy bills have had the biggest annual rise since the energy market was privatised.

Npower, the UK's fourth-biggest energy group, is increasing gas prices by 26 per cent and electricity bills by 14 per cent.

From yesterday, 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, British Gas having increased its gas prices by 35 per cent last month.

ScottishPower also announced that it had increased its electricity prices by 9 per cent; that will come into effect on Monday. The company's 1.2 million customers will pay £221 more a year on gas and an extra £38 for electricity.

Consumer groups criticised the energy companies and called on the Government to do more to combat fuel poverty. It is expected that this year five million households will spend more than 10 per cent of their income on energy.

Adam Scorer, of Energywatch, said: “An action plan must include: direct financial support extended to non-elderly low-income households, a mandate to force suppliers to deliver meaningful social tariffs, an end to discriminatory prepayment meter tariffs; and a commitment to achieve the sorts of energy efficiency that protects consumers in much colder European countries from the threat of fuel poverty.

“Six price rises from the Big Six' in just five weeks caps a miserable summer for consumers and highlights just how the companies appear to act in step with each other.”

Ofgem, the industry regulator, is to publish a report on prices shortly.

Tim Wolfenden, of uSwitch.com, the price comparison website, said: “The industry-wide shift to higher household energy prices is now complete. This is a heavy blow, and few households will emerge unscathed or unconcerned about the future affordability of their energy.

“Consumers should look to pay by direct debit and move to an online plan where possible to get the best available prices.”

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

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

The company, which haad already 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 that its electricity generation had also been affected by increases in the cost of raw materials.

Giuseppe Di Vita, managing director at 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, at 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.”

Further rises in domestic energy bills will put more pressure on the Bank of England to keep a lid on inflation which is currently at a 16-year high of 4.4 per cent. The Bank is announces 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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