Shares in Centrica, the parent company of British Gas, fell as much as 17% and were on track for their worst ever single-session performance on Thursday after a disappointing trading update.
The company, which said it was enduring "highly competitive" trading conditions, also admitted to a "disappointing" second-half performance in its business energy supply division.
Centrica also warned that its full-year earnings per share at 12.5p would be lower than market expectations due to a 0.8p impact from a one-off North American tax charge and reflecting a weaker performance at its UK business unit as well as warmer than expected weather in October and November.
Britain's biggest energy supplier lost 823,000 domestic customers between June and October - coinciding with its decision to increase its standard energy tariff by 12.5%.
The company said "collective switch" deals, in which large numbers of customers sign up to less expensive suppliers, had been responsible for much of the customer losses.