Centrica has today reported a loss of 823,000 energy customers in the four months to October 31, sending its shares plunging by over 20% at one stage today.
Britain's big energy suppliers are under pressure from smaller rivals able to offer cheaper deals.
"Trading conditions continue to be highly competitive and performance delivery since mid-year within the Centrica Business energy supply businesses has been disappointing," the company's chief executive Iain Conn said.
Centrica, which owns Bord Gáis Energy here, said it expected an adjusted operating cashflow of more than £2 billion, after a result of £2.69 billion last year.
Centrica's British Gas business had around 14 million residential energy supply accounts in 2016.
The sector has come under pressure since UK prime minister Theresa May said in October she would impose controls to tackle what she called "rip-off energy prices".
Home power bills have doubled in Britain over the past decade to an average of about £1,200 a year.
Centrica earlier this week said it would stop offering its standard variable tariff (SVT) - the payment targeted by the government cap - to new customers from March 31.
It also called on the government to phase out SVT deals altogether and prohibit so-called evergreen tariffs without an end-date, arguing that such deals reduce customer engagement.
