The telecoms regulator has fined mobile operator O2 £10.5m for overcharging customers over an eight-year period.
Ofcom said the company had identified issues with its billing processes in 2011 but efforts to address them were not successful.
As a result, the watchdog said, more than 250,000 customers were billed incorrectly between that year and through to 2019 while 140,000 actually paid the charges.
The problems centred on the way O2 was charging pay monthly customers who were leaving the provider – with many charged twice.
Ofcom’s enforcement director, Gaucho Rasmussen, said: “Mobile customers trust their provider to bill them correctly and fix any errors as quickly as possible.
“But these billing issues continued for a number of years without sufficient action from O2, and thousands of customers were overcharged as a result.
“O2 has refunded the customers who were affected, and we are satisfied the company has taken action to prevent this happening again.”
A spokesperson for O2 said: “As the operator proactively driving over £168m value back to our customers in the last year alone, we are disappointed by this technical error and sincerely apologise to customers impacted.
“As Ofcom have stated today, the vast majority of funds reported were not overpaid. Only 6% – £2.4m – relates to money that was overpaid by customers.
“We identified the issue ourselves and notified our industry billing auditor.
“We have also taken proactive steps to refund all impacted customers for the extra charges they paid, plus an additional 4%.”