British Gas owner Centrica has seen operating profits of £1.3bn in the six months to 30 June.
The figure was down 19 per cent on the same period last year.
The results include a 54 per cent fall in operating profits at its residential energy division, British Gas, to £270m.
Yesterday British Gas was fined £2.5m for mishandling customer complaints.
British Gas says higher wholesale gas prices are responsible for the fall in profits.
The news comes as the firm prepares to hike gas prices by 18 per cent and electricity prices by 16 per cent in August.
British Gas says it has been selling gas at a loss since April due to the rising cost of gas on the wholesale markets.
In its core business of home gas supply British Gas lost customers and market share compared with the same period last year.