British Gas owner Centrica has reported operating profits of £1.3bn in the six months to 30 June, down 19% on the same period last year.
The results include a 54% fall in operating profits at its residential energy division, British Gas, to £270m.
British Gas says higher wholesale gas prices are responsible for the fall in profits.
The news comes as the firm prepares to raise gas prices by 18% and electricity prices by 16% in August.
The increase will push up the average bill for around nine million customers by £190 a year.
Richard Lloyd from consumer group Which? warned that customers would be angry.