The last of Britain's big six energy suppliers increased their prices on Friday. Npower, the UK's fourth-biggest provider, raised gas charges by 26 per cent and electricity by 14 per cent, meaning that its 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 lifted its gas bills by 34 per cent, the second-largest rise after British Gas increased its prices by 35 per cent last month. It also announced that it will increase its electricity prices by 9 per cent from Monday. The company's 1.2 million customers will pay £221 more a year for gas and an extra £38 for electricity.

The increases could push more people into fuel poverty, with five million households now predicted to spend more than 10 per cent of their income on energy this year.

Consumer groups criticised the companies and called on the Government to take action. Adam Scorer, director of campaigns at Energywatch, said: “Six price rises from the big six in just five weeks caps a miserable summer and highlights how the companies appear to act in step with each other.”

Ofgem, the industry regulator, is due to publish a report into prices next month.

Like ScottishPower, npower is blaming the rises on huge increases in wholesale costs, which it claims have made its previous prices unsustainable. Giuseppe Di Vita, its managing director, said: “I am 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: “We understand the financial impact this announcement will have on our customers. Although we are one of the last companies to announce increases, we are sorry we couldn't hold on any longer.”

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 its announcement, Scottish and Southern Energy, the country's second-biggest supplier, with 8.5 million customers, said that it was raising electricity and gas prices by 19 per cent and 29 per cent respectively.

In July, Centrica, the owner of British Gas, announced a 35 per cent rise in the cost of gas and 9 per cent in electricity after profits at its residential business fell by 69 per cent in the first half. This means that its 16 million customers face paying £261 more a year, taking the average bill to £1,305.

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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