British Gas is to be investigated by energy regulator Ofgem for potentially misleading customers over fees for switching 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 told to pay up to £60 for switching, despite being within the 49-day period.
It is not known whether anyone actually paid the fees.
British Gas said it would co-operate with the inquiry, but gave no further comment.
Ofgem said that it would also be looking at whether British Gas had obeyed the rules which oblige them to write to customers on fixed deals, telling them they are about to expire.
However it said the opening of the investigation did not imply that they had made any findings of non-compliance.
MoneySavingExpert said that it had received similar complaints about Npower at the end of last year, and subsequently about E.On.
Ofgem said it was talking to both companies about their exit fees, but they were not part of the inquiry.
News of the investigation was welcomed by MoneySavingExpert founder Martin Lewis.
“The rules are very plain; you cannot and should not be charged exit penalties if your switchover takes place within the last 49 days of your energy fix,” he said.
“At least two firms – British Gas and Npower – have wrongly put that they would charge in their official literature. At best they are careless in the way they treat customers; at worst that they are trying to bully them into staying with misinformation.”