British Gas is to be investigated by energy regulator Ofgem for potentially overcharging customers who wanted to switch to other providers.
Those planning a switch before a fixed-term deal expires can do so for free for up to 49 days before the deal ends.
But Ofgem is to examine allegations that British Gas told some customers they would have to pay a termination fee within that period.
The complaints were passed to Ofgem by the website MoneySavingExpert.
The website said it had received complaints from some customers that they had been charged up to £60 for switching, despite being within the 49-day period.
British Gas said it would co-operate with the inquiry.
Ofgem said that the opening of the investigation did not imply that they had made any findings of non-compliance.