O2 customers are being urged to check their bills after thousands of people have reported being overcharged.
The charges, which have ranged between £3 and £13, have reportedly been appearing on customers statements as a 'loyalty discount'.
The mobile network claims that it accidentally applied a 'loyalty discount' for an extra month, causing some customers to pay less than usual. Its billing system then automatically added a charge the next month to claim the discount back - resulting in customers being over charged.
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As an apology, O2 has said it will write to all customers affected and extend the loyalty discounts for a further 12 months, as long as customers remain on the same tariff.
Customers are being asked to check their bills and contact O2 if they have been affected to confirm whether the extended discount will be applied.
These loyalty discounts vary depending on the customers tariff and circumstances.
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The error was first reported by MoneySavingExpert, but customers have been taking to social media to complain about the charge.
One customer said: "Hey @O2 just wondered why my bill has gone from £21.40 a month to over £50??
"Ever since I signed up for my sim only plan I get a £12.50 loyalty discount off my bill but this month you've added £12.10 loyalty 'discount' to my bill?? Am I paying you for my loyalty??"
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Another added: "Thanks @O2 for the loyalty discount... not sure if I should stay loyal if I'm going to be charged for it."
A spokesperson from O2 said: "We're aware that some customers who'd been receiving a discount for 12 months had received a further discount for one additional month.
"Unfortunately this was a mistake and on the next bill, our billing system automatically applied a charge for the amount that had been mistakenly discounted.
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"This had appeared on the itemised bill as 'Loyalty Discount' giving the impression we had charged for a discount.
"Once we identified what had happened, it was quickly fixed."
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