Energy group SSE has said it will increase tariffs by 9% on average in a move impacting around five million electricity customers and 3.4 million gas customers.
SSE, which will introduce the bill hike on 15 October, 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.
SSE is the first of the "big six" to raise prices this year, but it is thought others might follow suit soon, with British Gas parent Centrica recently warning wholesale price rises may lead to higher bills this autumn.
Utility groups are also facing increasing costs of regulation and government-sponsored schemes, which are being passed on to customers.
Martin Lewis of MoneySavingExpert.com warned of a further energy price misery to come.
He said: "When one moves others follow, and over the next three months I'd expect to see similar announcements."
He urged customers to consider switching to better deals and said there were some good fixed rate deals on the market.
Richard Lloyd, executive director of Which?, called on the government to reform the energy market ahead of the expected raft of price rises.
He said: "We can't go through another winter with people worrying about their energy bills. The government and the regulator must reform our broken energy market.
"It's time for energy prices to be properly transparent and tariffs to be made simpler, so that consumers get a fair deal."
SSE's price rise comes after it reduced gas prices by 4.5% in March.
It last increased gas prices by 18% and electricity tariffs by 11% last September.
SSE's recent results showed its domestic supply operation made £271.7 million in the year to 31 March, although this was 21% lower than a year earlier.
Consumer Focus said that, following the October price rise, SSE will be by far the most expensive of the big six providers for the typical dual fuel tariff.
Audrey Gallacher, director of energy at Consumer Focus, said: "This price rise will be a hard blow for consumers in the current difficult economic climate.
"People will be worried about a run of price rises, but we see little evidence in the trends in wholesale prices or in the performance of companies that would justify all suppliers following suit."