The average price of gas and electricity from energy supplier SSE is set to increase by an average of 9% from the 15 October.
Up to five million electricity, and nearly 3.5 million gas customers will be affected by the price rises, with the average monthly bill increasing by £8.53 a month.
SSE has defended the price rises, stating that increases in the cost of energy on the wholesale market have hit a level where it has to pass them on to customers.
Prices will be capped again from October until at least the second half of 2013, according to the company.
SSE previously pledged to keep prices the same until October 2012, after it cut gas prices by 4.5% in March 2012.
Increases in cost
"Unfortunately, the increases in costs that we have seen since making this pledge can no longer be absorbed and mean that we are unable to keep prices at their current levels beyond this autumn. An increase in our prices has therefore, regrettably, become unavoidable," says Ian Marchant, SSE chief executive.
Customers affected by the rises will receive a letter at least 30 days before the price hikes take effect.