The owner of British Gas has lost more than a quarter of a million customers since the beginning of the year despite not raising prices.
Centrica reported a 261,000 fall in residential accounts to 14.2 million since January, some of which was due to fixed-term contracts ending.
It also said warmer than expected weather had hit energy consumption.
The firm also criticised a government proposal to cap energy prices, warning that it could lead to higher bills.
The Conservative Party has pledged to cap prices if it wins the general election on 8 June.
“Centrica does not believe in any form of price regulation. Evidence from other countries would suggest this will lead to reduced competition and choice, and potentially higher average prices,” it said in a trading update on Monday.
“We have had a regular and constructive dialogue with the Government and have proposed alternative ways to improve the market further and address their concerns, without resorting to price regulation.”
Centrica remains the UK’s biggest domestic energy supplier.
The company said it expected to cut 1,500 jobs this year and generate cost savings of £250m in addition to the £384m it reported in the previous year.
Chief executive Iain Conn will face shareholders at the company’s annual meeting on Monday. Some are angry that his pay rose by about 40% to £4.1m last year.
British Gas said in February that it would freeze its gas and electricity prices until August, in contrast to the likes of Scottish Power, Npower and EDF Energy, which all announced higher tariffs.