From tomorrow customers who take both gas and electricity from E.On will see their annual bills rise by by £227 to a total of £1226.
The company, which has 5.5 million UK customers, blamed soaring wholesale gas costs for its decision to raise gas and electricity bills.
Managing director Graham Bartlett said: "I'm very aware of the effect that today's announcement will have on our customers and I recognise that this is a very tough time for everyone."
"This was not an easy decision to make and we've tried to keep these increases as low as possible while protecting as many of our customers as we can."
E.On said around 1.4 million of its customers on price protection and fixed tariff products would be unaffected by the higher bills.
The move comes a day after wholesale gas prices jumped more than 14% when a leak on a North Sea pipeline prompted fears about supplies this winter.
Norway's gas producer Statoil Hydro said it discovered the leak on a pipeline linking its Kvitebjoern field to an onshore processing plant. The pipeline could remain shut until next spring.
The E.On rise follows British Gas's announcement of a 35% rise for gas bills last month, and 9% for electricity. EDF bills rised by up to 22%.
The UK is highly exposed to gas supply disruptions because it lacks gas storage compared with its European neighbours.
Historically, the UK has benefited from lower gas prices during the summer due to domestic overproduction and easily available imports.
But with hydrocarbons such as gas and oil now viewed as a scarce resource, there has been more intense competition for supplies from the continent.
clickyObviously not as big a increase as BG, but where does this leave all the people to who moved to E.On after BG increased their prices last month.