Although rising energy bills have been a fact of life for some time, the decision by British Gas this week to lift gas charges by a breathtaking 35 per cent has caused an outcry, raising fears that millions of households will slide into “fuel poverty”.

Unveiling half-year profits of £994m on Thursday, parent company Centrica was at pains to point out group profits had fallen 17 per cent and British Gas profits fell 69 per cent because of wholesale gas costs.

But the size of the profit and the fact Centrica raised its dividend by 16 per cent was seen by consumer groups as rubbing salt into customer wounds.

Speaking to the Financial Times, Sam Laidlaw, Centrica’s chief executive, said his group was not making excessive profits. The rise in wholesale gas prices by as much as 90 per cent over the past year meant that higher bills were inevitable.

“By the second quarter, British Gas Residential was breaking even and going into loss-making territory. If we are going to invest in new power stations and keep customers’ homes lit and warm we have to pass through some of that increase in costs,” he said.

Centrica’s interim profits may have fallen 19 per cent but politicians shed few tears of sympathy, writes Jim Pickard.

Labour MPs in particular are angry that the owner of British Gas made almost £1bn in profits – 24 hours after announcing a jump in the price it will charge households.

There is growing political pressure in favour of a tax to punish energy companies. It would be welcomed by the trade unions and could – if the proceeds were put towards helping consumers – go down well with the public.

The government floated the idea of a windfall tax earlier this year, although it shied away from such a decision in the Budget.

But some observers believe Gordon Brown may take action in the autumn in a bid to revive his ailing premiership.

On Thursday the Local Government Association called for a new £500m annual charge on energy suppliers to combat fuel poverty and cut carbon emissions.

There is a divergence, however, between leftwingers calling for a tax on energy profits and other politicians advocating a more modest windfall tax to retrieve the £9bn made by energy companies through the European Emissions Trading Scheme (ETS).

On Thursday Vince Cable, Liberal Democrat Treasury spokesman, made the case for an ETS levy.

“There is a strong case for retrieving that money for customers, through introducing a decent social tariff system . . . and investing in radical energy saving measures,” he said.

On Monday the Commons’ business select committee called for a one-off ETS levy.

But Peter Luff, chairman of the committee, said his report should not be misinterpreted as a call for a a levy on excessive profits.

Following the 35 per cent price rise, British Gas Residential’s profit margins for the second-half will probably be about 4 per cent, the same as the first half.

The public’s suspicion that Centrica and other suppliers are using high gas costs as an excuse to ramp up profits is fuelled partly by the fact the companies refuse to disclose how much they pay for their gas.

“At one stage we did publish the prices we paid, but we were the only company that did so and this put us at a competitive disadvantage,” Mr Laidlaw said.

Centrica and its rivals are being forced to supply detailed gas trading information to Ofgem, the energy regulator, as part of its investigation into the UK’s energy market.

The only measure that Centrica does disclose is the commodity costs for BG Residential, up 39 per cent to £2.25bn in the first half.

High gas prices lifted profits at Centrica’s upstream business by 50 per cent, leading to more charges that it is profiting as consumers suffer.

But Mr Laidlaw said there was little scope to cross-subsidise BG Residential’s bills as the upstream division is heavily-taxed. “The government won’t let us cross-subsidise,” he said.

The government is under pressure to relieve the burden of rising energy costs, and there have been calls for a windfall tax. Mr Laidlaw admits this is a “politically-charged time”, and notes that up to a quarter of British Gas customers are now classed as “fuel poor”.

“A straight windfall tax would be a mistake as it would damage investor confidence at a time the industry needs to spend £100bn on renewable energy alone.”

Centrica has suggested instead that the government forces power companies to pay for carbon permits they receive free under the European Union’s emissions trading scheme, and use the proceeds to help customers.

Whether Ofgem recommends big changes to energy markets when it reports to government in the autumn, it is clear international oil and gas prices will have an increasingly large influence on UK energy bills.