E.ON became the latest utility to raise its energy prices in the UK last week, with electricity up 11.4% and gas up 18.1%.

However, in concession, the utility announced that dual fuel customers would only see a rise of 15.2% from September 13, while just under 600,000 customers – including the most vulnerable on WarmAssist tariffs – will be unaffected.

E.ON, which serves 7.4 million customers in the UK, has also promised not to raise prices for its Age UK Energy customers until at least September 2012 and will launch a two-year, no premium, no exit fee fixed price deal for existing customers.

Graham Bartlett, managing director of E.ON’s energy solutions business admits it is hard for customers but blamed volatile wholesale prices.

“Uncertain times have had a huge effect on wholesale prices, with events in Japan and Libya having a dramatic effect on gas and power prices in a relatively short period of time,” he says.

E.ON quoted Ofgem figures, which report wholesale prices up around 30% compared with last year. Scottish and Southern Energy, Scottish Power and British Gas have also announced substantial price rises, while Good Energy is increasing its gas prices.

But consumer group Which? says that while customers will be unsurprised by E.ON’s actions, energy companies need to do more to help improve energy efficiency.

“There is a real risk that millions will struggle to heat their homes this winter, so it’s crucial that suppliers do more to help their customers get on the cheapest available tariff, the most economical payment method, and to take up free energy efficiency offers,” says executive director Richard Lloyd.