An investigation by the regulator found an error in Giffgaff’s billing system led to around 2.6 million customers being overcharged by up to £2.9m.

Giffgaff, which is owned by Telefónica UK, overcharged customers who bought ‘goodybag’ bundles using their pre-paid credit.

‘Goodybags’ are pre-paid bundles of voice minutes, text messages and data.

A delay in Giffgaff applying the bundle purchase to customer accounts meant any voice calls customers made, or the data they used at the time, came out of their pre-paid credit. These services should have been free immediately from the point the bundle was purchased – so customers were effectively charged twice.

Giffgaff applied the ‘goodybag’ bundle to a customer’s account only once they ended the voice call they were on, or when they started a new data session – for example, by turning their phone off and back on again.

Giffgaff said it has already refunded around £2.1m to affected customers.

If you are a Giffgff customer and think you are still owed a refund, you might wish to contact the company.