British Gas-owner Centrica said that its UK home operating profit fell to £516m in the six months to 30 June.
Residential customers numbers fell by 3%, or 399,000, which Centrica blamed on a "competitive environment" and the end of long-term contracts.
Total revenue fell 13% to £13.3bn and operating profit fell by 12% to £853m.
Commenting on the UK's vote to leave the European Union, Centrica said it expected the direct short-term impact on the company to be limited.
"However, the result creates general uncertainty which adds to challenges for UK businesses in all sectors, with implementation plans as yet unclear," it said.
Centrica said: "The UK is now a major energy importer and what happens in the European energy market will ultimately impact energy consumers in the UK. We will continue to engage with the UK Government and the European Commission as we move forward."
The company said that while residential customer numbers fell in the first half as a whole, they returned to growth in June after it introduced a 5% cut to domestic gas bills in March. British Gas was one of the last of Britain's "big six" energy suppliers to announce cut prices this year amid low wholesale gas prices.
While Centrica's operation that supplies energy to UK businesses returned to profitability in the first half, the company said its North American division was "significantly impacted" by record warm weather in the US compared with extreme cold in the first half of 2015.
Centrica said it had cut costs by £141m during the first half of the year as part of a strategy to make £750m in savings by 2020. This includes shrinking its headcount by 6,000 and Centrica expects to achieve 3,000 job cuts by the end of this year.
Shares in Centrica rose 1.6% to 245p in early trading as it struck an upbeat note on meeting its growth targets, despite factors such as a lower oil price.
Centrica chief executive Iain Conn said: "The first half of the year has been demanding for Centrica, but the response has been strong and I am encouraged by the progress we have made.
"I remain confident in our ability to deliver both attractive returns and underlying cash flow growth, as we continue to implement our strategy."
Profits at British Gas fell by 7% in the first half of the year as nearly 400,000 customers switched energy suppliers.