British Gas owner Centrica has lost 261,000 domestic customers so far this year, but added it was on track to hit its full-year targets.
The UK's largest energy supplier said the shortfall reflected its decision to end so-called "collective switch" deals, where third party firms negotiate a better deal for a group of customers.
Centrica also added its output was hit by warmer than expected weather in the UK and the US, in a trading update covering the start of its financial year from February.
The group, which serves more than 11 million homes and businesses, also noted that wholesale oil and gas prices have fallen since the group's full-year results three months ago.
However, the group said it was on track to make £250m ($325m) of savings this year, while making capital investments of "no more than" £1bn. It also expects to cut around 1,500 jobs this year, as part of its ongoing round of reductions.
Centrica chief executive Iain Conn said: "Customer service is improving, we have launched new offers delivering choice for customers and rewarding loyalty and we continue to develop our technology capabilities. We remain on track to deliver against our 2017 targets."
The "big six" energy firms – Scottish Power, SSE, British Gas, EDF, Npower and E.ON – have been battling to retain customers following a surge in independent, challenger brands. There are now over 40 domestic energy suppliers to homes.
However, the Conservative Party is expected to include proposals for a cap on expensive standard variable energy tariffs – the most common rate paid by households – in its forthcoming manifesto.
"Centrica does not believe in any form of price regulation", the power group said in its update. "Evidence from other countries would suggest this will lead to reduced competition and choice, and potentially higher average prices."
"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."
Scottish Power, among other major suppliers, has also said that the introduction of price caps will hurt consumer competition and choice.
However, the market had been expecting a clampdown on some tariffs after most of the "big six" energy companies announced price hikes earlier this year, sparking outrage from MPs and consumer groups.
These groups complained that consumer prices for gas and electricity rose, despite wholesale prices remaining lower than they were in 2014.
British households have spent £7bn more than they should have on energy bills over the last three years, small energy supplier First Utility said in March.
The idea of a cap on household energy tariffs was first put forward by former Labour leader Ed Miliband in the run up to the 2015 general election.