British Gas owner Centrica lost 340,000 customer accounts in the UK in the first half of this year, group chief executive Iain Conn has told the BBC.
His comments came as Centrica said operating profits at its consumer business fell 20% to £430m in the six months to the end of June.
Mr Conn said he hoped customer numbers would stabilise but would not give a timeframe for when this might happen.
British Gas still has 3.5 million customers on standard variable tariffs.
This is down from 4.3 million at the start of the year.
by BBC business editor Simon Jack
The continuing fall in customer numbers and the number of services they buy will be a disappointment to the company that has been trying to stem the decline by offering new services, including allowing customers to monitor and manage their energy usage through mobile phone apps.
That push towards connected services has seen some growth but if the acid test is whether customers want to stay with you – British Gas is failing.
Centrica is much more than just British Gas, of course – it also has businesses in the US and Europe but overall performance has been roughly flat.
Chief executive Iain Conn is coming under pressure from shareholders to improve the performance of the company. The share price has more than halved in the five years that he has been in charge.
He points to a collapse in oil and gas prices since 2013, ongoing regulatory probes, the expense of smart meter roll out and now the introduction of a price cap.
Despite this, he is convinced that growth is around the corner.
He will want that corner to come soon.