Scottish and Southern has become the third of the 'Big Six' energy companies to announce price hikes for gas and electricity.
The rises will add around Â£171 to the average dual fuel bill for its customers - as electricity leaps up 11% and gas by 18% in September.
Scottish Power and British Gas have already announced their rises.
Scottish and Southern says three main factors have prompted the rise - which they say will be the last until at least August 2012.
It has blamed Ofgem for raising the cost of the networks for distributing energy by 14% inÂ a year, as well as rises in wholesale energy costs, and the cost of implementing mandatory environmental and social schemes.
The rise will mean Scottish and Southern customers face their highest bills ever, as rises will mean the average leaps from Â£1,094 to Â£1,265.
With all of the Big Six expected to announce rises in the coming months, Government energy minister Chris Huhne has already urged the public to compare energy prices to get cheaper gas and electricity.
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