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.