BT has been censured by Ofcom for charging rival companies too much for sending out engineers to repair faulty broadband lines.
The watchdog agreed that BT appeared to be billing more than the actual time worked by engineers.
This, it said, "suggests there is a risk of cost over-recovery as a result of its approach", the Daily Mail reports.
As a result, BT could be set to pay a considerable sum in repayments to those affected.
Both TalkTalk and Sky have insisted that BT's Openreach infrastructure division has overcharged them when engineers were deployed to resolve faults on the network.
"Although not all engineering charges are passed on to consumers, a good proportion of this overpayment will have been borne by customers unnecessarily," a TalkTalk spokesman said.
"To make matters worse, these services mostly affect non-BT customers. Most of the time, BT Retail uses completely different engineering services."
Openreach has stressed that it has followed Ofcom's guidelines to set its prices. As a result, it is "disappointed" that the watchdog has assessed the case in a way that "differs from our understanding of that guidance".
The organisation added that the pricing issue is "detailed and complex".
This comes amid the ongoing debate over Ofcom's proposed reforms to Openreach.
BT has been ordered by the regulator to give more independence and investment powers to Openreach, with the broadband subsidiary being run as a distinct and legally separate company with its own board.
The watchdog believes its proposals will provide Openreach with the greatest possible degree of independence without separating the companies entirely, and lead to decisions being taken for the good of its customers and the wider telecoms industry.