Image caption SSE said the price rises were regrettable but unavoidable

Energy firm SSE, which trades as Scottish Hydro, Swalec and Southern Electric, will increase its domestic gas and electricity prices by an average of 9% from 15 October.

It blamed the increases on the extra cost of using the gas and electricity networks and rising costs in energy wholesale markets.

Some 3.4 million gas and five million electricity customers will be affected.

However, there will be no more price rises before the second half of 2013.

The forthcoming increases mean that a customer with an average standard dual-fuel bill will pay an extra £102 for the year, or £1,274 in total.

The company's chief executive Ian Marchant, said the forthcoming increases were regrettable but unavoidable.

"We pledged last summer to cap our energy prices for as long as possible and until at least August 2012, and then in January extended this pledge to October 2012," he 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," he added.

In May, the company reported a 2% rise in annual pre-tax profits to £1.33bn, though profits in its division which supplies electricity and gas to homes and businesses fell 20% to £321.6m.

Energy market 'broken'

In 2011, all the big-six energy suppliers raised their prices, in some cases twice.

But earlier this year they all staged a round of small price cuts, affecting either their gas or their electricity customers.

SSE cut its gas prices by 4.5% in March this year.

However, in May, Centrica - which owns British Gas - warned that continued increases in the wholesale price of gas might lead to renewed domestic price rises this autumn.

Richard Lloyd, of the consumers' association Which?, 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."

Martin Lewis, of the consumer website MoneySavingExpert.com, said many people could save money by switching to one of the fixed-rate deals currently on offer.

"Two major energy companies are currently offering cheap fix deals at the same price as the cheapest variable deals, cutting many people's bills by hundreds of pounds and guaranteeing no price rises for up to two winters," he said.

"More surprisingly, those tariffs do not involve exit penalty lock-ins, so you can leave whenever you want."