Telecoms giant BT wrongly charged customers with a limited broadband package for exceeding their usage cap in June, MoneySavingExpert.com has learned – and the firm is now writing to those affected to tell them they will be refunded.

The overcharging, which occurred between 22 and 30 June, affected those with a data limit on their broadband package, typically 10GB, 20GB or 40GB. BT refuses to give a figure for how many customers were overcharged or even how many customers it has on limited broadband packages, but insists "a small number" were affected.

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What went wrong?

The overcharging emerged after one MoneySaver, who asked not to be named, received an email earlier this week from BT, which apologised because the firm "accidentally charged you too much".

When we contacted BT, it told us that "a glitch in our systems meant some customers would have been accidentally charged a few pounds too much". BT's standard extra usage charges are £1.50 per GB.

What happens if I've been overcharged?

BT says there's no need for anyone to do anything – if you've been overcharged it says you'll have been emailed. Those who have had money wrongly taken will have a full refund automatically added to their account and this should show up on the next bill they receive.

But if you're concerned or worried money may have been wrongly taken, it's worth checking your bill for June – if you have any questions you can contact BT on 0800 800 150. If you're unhappy with the overcharging and want to complain to BT, you can do so using the free online complaints tool Resolver* which helps you draft, manage and, where necessary, escalate your complaint.

BT won't say if all of its customers with limited broadband packages have been overcharged, but customers on unlimited broadband packages aren't affected.

What does BT say?

A spokesperson for BT says: "BT is writing to a small number of broadband customers to apologise for a mistake which meant they were wrongly charged for going over their usage limit in June. All of these customers will be refunded for the amount we overcharged without them having to do anything."

If you've been affected by the glitch let us know in the comments below or by emailing news@moneysavingexpert.com.