British Gas has admitted losing 200,000 customers since it raised household energy bills in the summer and warned its profits will be hit by recent warm weather.
A day after announcing 850 job losses in the company's service business, British Gas parent Centrica said its earnings for 2011 will be marginally lower than expectations.
It blames "unusually warm weather experienced in the UK in recent months" rather than the customer drop-off.
The British Gas residential business had been tipped to record profits of £600m.
It made £272m in its first six months but the company says the period between April and August was loss-making.
British Gas and the other so-called 'big six' suppliers raised their prices in the summer to take account of higher wholesale energy costs - British Gas by an average of 18% for gas and 16% for electricity.
The firms have consistently denied profiteering in the face of a storm of protest from consumer groups.
Energy Secretary Chris Huhne had accused households of laziness in their approach to shopping around for the best tariffs.
Centrica suggests that a 17% fall in residential gas usage in the first ten months of the year compared to 2010 may also be down to greater roll-out of energy efficiency measures within homes - such as cavity wall and loft insulation.
It says wholesale gas costs are 26% higher this winter than last but it has ruled out any further bill increases this year.