Shell Energy is being investigated by the communications regulator for not alerting customers that their broadband contracts were coming to an end.
The company said that it had alerted Ofcom to problems in how it communicated with customers.
The regulator on Friday opened an investigation into whether the energy giant complied with its rules on communications with customers.
Under rules introduced a little over two years ago, providers of broadband, mobile, home phone and pay-TV have to warn their customers when their minimum contract period ends.
They must also be told how much they could save if signing up to a new deal, and be reminded of the best deals every year.
“Information provided to Ofcom by Shell Energy indicates that it did not send these notifications to some customers for a period of time starting in February 2021, while other customer letters may not have contained all of the required information,” Ofcom said.
“We aim to complete our evidence-gathering phase by the end of summer 2022. We will publish a further update on any next steps once that phase is complete.”
A Shell Energy Broadband spokesperson said: “We take our customer responsibilities very seriously which is why we alerted Ofcom to an issue where we did not send communications to some of our customers regarding the end of their contract, although the price they paid was unchanged.
“We’ve since addressed the issue and sent appropriate reminders to the affected customers.
“We look forward to working with Ofcom to outline the improvements we’ve made and assure them of our commitment to delivering an excellent customer experience.”