Energy group SSE says it will increase tariffs by 9% on average in a move impacting around 5 million electricity customers and 3.4 million gas customers.
SSE will introduce the bill hike on October 15 and said it comes in response to energy price increases in wholesale markets, as well as rising costs of using the National Grid network.
The UK's second largest energy firm, formerly known as Scottish & Southern Energy, said it will cap bills following the October price rise until at least the second half of 2013.
SSE said the 9% rise, which applies across gas and electricity, will add another £8.53 a month on to the typical monthly direct debit, dual fuel customer - taking the average annual bill to £1,274. The group committed in January to keeping household bills on hold until October.
Ian Marchant, SSE chief executive, said: "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."
SSE, which trades as Southern Electric, Swalec and Scottish Hydro, said it had seen a 14% increase year-on-year in the average price in the wholesale market to secure gas for the coming winter.
The group also announced changes to simplify bills by introducing a new fixed standard charge of £100 per year per fuel and a single unit rate for energy used.
It will offer fixed discounts off this price, which will see direct debit customers pay £40 less a year for each fuel, while prompt quarterly bill payers will get a £20 discount per fuel and those choosing paperless billing will get an extra £6 a year per fuel.
The group said it would ensure around 400,000 low-usage customers can remain on a no standard charge tariff if they wish, otherwise they could see their bills rise as a result of the changes.
