Consumer groups have reacted with fury after energy firm SSE confirmed plans to raise its gas and electricity bills by an average of 9% from October 15.
The company, formerly known as Scottish & Southern Energy, blamed "sustained increases" in the cost of using the electricity and gas networks, costs associated with mandatory Government schemes and the price it had paid for energy in the wholesale markets.
Wholesale gas charges had risen 14% year-on-year, the group said, its statement pointing to the fact SSE had met its pledge not to raise existing household tariffs until at least October 2012.
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The company added, as a result of the looming price increases, it would implement a new guarantee on pricing, saying it was now committed to cap household energy prices at the new level until at least the second half of 2013.
SSE said the 9% rises would 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 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.
Consumer groups were quick to condemn the price increases.
Which? executive director Richard Lloyd 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."
Ann Robinson, director of Consumer Policy at uSwitch.com, said: "This is a serious blow for cash-strapped consumers. With winter on its way, households will now have to brace themselves for higher fuel prices too.
"Last winter, over eight in 10 households rationed their energy use because of cost - a round of price hikes this winter will condemn many more to this misery too."