UK internet service providers (ISPs) TalkTalk and Tiscali have come under fire from Ofcom for breaching telecoms rules.
According to the media regulator, the ISPs have mistakenly been continuing to charge some customers for broadband services after cancellation.
Ofcom said it had received more than 1,000 complaints this year alone over incorrect charging, and has warned that enforcement action will be taken if the situation is not resolved quickly.
TalkTalk - which last year bought Tiscali - has been given until December 2nd to comply with a General Condition notification, or face a potentially hefty fine.
The firms have also been ordered to provide refunds to affected consumers where applicable, stop debt collection procedures and take appropriate steps to repair the credit rating of affected customers.
Ofcom's Director of Consumer Affairs Claudio Pollack said: "Ofcom is determined to stand up for consumers and take action against companies that break the rules.
"Our investigation into TalkTalk and Tiscali UK found that they had billed customers for cancelled services; this is unacceptable which is why we have ordered them to clean up their act or face the consequences."