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Giffgaff has been fined £1.4 million by regulator Ofcom after the mobile phone network made "unacceptable" billing mistakes seeing it overcharge around 2.6 million customers.

Ofcom said Giffgaff - which is owned by O2 parent Telefonica - overcharged customers up to a total of almost £2.9 million.

Gaucho Rasmussen, director of investigations and enforcement at Ofcom, said: "Getting bills right is a basic duty for every phone company.

"But Giffgaff made unacceptable mistakes, leaving millions of customers out of pocket.

"This fine should serve as a warning to all communications providers: if they get bills wrong, we'll step in to protect customers."

Giffgaff customers who bought a ‘goodybag’ bundle, while using their pre-paid credit, were overcharged, Ofcom said.

The reason was a delay in Giffgaff applying the bundle purchase to customers accounts, which meant calls and data they were using at the time came out of their pre-paid credit.

Giffgaff only 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 on again.

Ofcom said the services should have been free immediately from the point the bundle was purchased – so the customers were effectively charged twice.

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

In cases where it hasn’t been able to find the customers to refund, it has donated the money to charity.

If you’re a Giffgaff customer and think you are still owed a refund, you might wish to contact Giffgaff.