At the same time, millions of families are rationing their use of heat and light for fear of running up bills they will not be able to afford.
The unusually warm weather in recent weeks has also allowed struggling families and pensioners to delay putting on their central heating.
The combination of fewer customers and lower energy sales has forced the firm’s parent company, Centrica, to warn that profits will be below expectations.
The number of customer accounts at the UK’s biggest energy supplier is down by about 100,000 to 15.9million since the start of the year, according to City analysts.
The bulk of these are believed to have left since the company announced price hikes in the autumn.
Centrica said average household gas use is down by a whopping 17 per cent compared with a year ago, while electricity is down by 3 per cent.
Business consumption fell 15 per cent for gas and 12 per cent for electricity. Despite this, City analysts predict the company will make a pre-tax profit of £2.3billion this year, which would represent a rise of 20 per cent or £400million on 2010. Rebecca Seabury, an energy analyst at Inenco, said: ‘I’m sure there won’t be many tears amongst consumers over these figures.
‘Price rises announced by British Gas and the other energy companies have put millions of households into fuel poverty.
‘If the extreme cold weather of last winter is replicated this year then British Gas profits will rise again but for many it will be a stark choice between eat or heat.’
Cheaper deals are available from British Gas’s rivals, but all the ‘big six’ suppliers have raised prices sharply in recent weeks.
Mark Todd, director of the price comparison service Energyhelpline.com, called on British Gas to cut prices in order to end the customer exodus.
‘Consumers want low cost tariffs and any company that offers these will gain lots of new customers,’ he said. ‘Currently, the rate of switching through price comparison services is 50 per cent higher than normal.
‘Energy prices are now higher than at any time in UK history.
‘The industry regulator, Ofgem, says that the average UK energy bill is currently £1,345 a year. People simply can’t afford this huge amount.’