British Gas owner Centrica lost 110,000 energy supply accounts in the first four months of the year.
That is roughly equivalent to 70,000 customers as many households buy their gas and electricity from British Gas, so will have two accounts.
Last year, the company lost 1.3 million energy accounts.
Centrica said there had been “high levels of competitive intensity”, but said the rate at which it has been losing customers had been slowing.
The company has almost 13 million energy supply accounts in the UK.
Many of those used more energy during the spell of extreme cold weather earlier this year, which was branded the Beast from the East.
Centrica said it had seen “increased energy demand” during that period. However, the weather also caused an “exceptionally” high number of boiler breakdowns.
In one week Centrica fixed 145,000 breakdowns – more than twice the normal weekly number.
It said the cost of those extra callouts was expected to hit results at its service unit in the first half of the year.
In April, British Gas announced a 5.5% increase in both gas and electricity bills, which comes into effect at the end of the this month.
It blamed the rising wholesale cost of energy and the cost of meeting emissions targets and introducing smart meters.
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Other big energy firms have also announced price increases this year, including Npower, EDF and Scottish Power.
Rising energy prices have spurred record numbers of customers to switch suppliers.
In the first three months of the year, 1.3 million customers switched energy supplier – a record quarter, according to Energy UK.
That intense competition was partly behind 4,000 job cuts announced by Centrica in February. It also blamed the planned introduction of an energy price cap.
About 12 million households are on some form of uncapped default tariff, which can cost hundreds of pounds a year more than the cheapest deals.
The government wants to cap those tariffs and legislation to do that is currently working its way through Parliament.