Centrica, the parent company of British Gas, has seen its share price fall by more than 2% after the energy giant lost 224,000 customers in the first three months of 2016.
British Gas is still the UK's largest supplier of residential gas and electricity with 14.4 million customers, but it lost 1.5% of its customer base in just the first quarter, Centrica said in its trading update.
Centrica said the majority of those leaving were doing so at the end of a fixed-price contract, rather than cancelling their contract halfway through the term.
Just under a year ago, British Gas announced it was spending £50 million in an attempt to reduce the number of customers leaving for another supplier.
The company also said it was around a third of the way through planned job cuts for the year. Last year, British Gas announced 3,000 job cuts as part of a five-year savings plan worth £750m. Centrica said it has so far cut 800 jobs.
The news of British Gas losing a considerable number of customers comes a month after the UK's competition regulator, the Competition and Markets Authority (CMA), said millions of customers could see savings on their energy bills if they switched supplier. But the report also found that more than a third (35%) of UK consumers have not considered switching supplier.