British Gas saw a 7% fall in profits in the first half of the year as it lost almost 400,000 customers.
Owner Centrica announced its UK domestic operating profits fell to £516 million in the first six months until 30th June.
Around 3% or 399,000 of its customers switched to rival suppliers during that period.
The group’s total revenue dropped 13% to £13.4 billion, with adjusted operating profits at £853 million, a 12% fall.
Exploration and production was down 7% but nuclear generation remained strong.
Iain Conn, Group CEO 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. We are delivering underlying performance improvement and are building a robust platform for customer-focused growth.
“I remain confident in our ability to deliver both attractive returns and underlying cash flow growth as we continue to implement our strategy.”
Centrica added the uncertainty following Brexit “may lead to volatility in markets with potential fluctuations in foreign exchange rates, interest rates and commodity prices”.
Its report stated: “The Group’s focus is now on understanding what the result means for energy and other material business regulations over time and how this would impact the competitiveness of the European energy markets. The UK is a major importer and what happens in the European energy market will ultimately impact energy consumers in the UK.”