A "processing error" has been blamed for customers charged twice while paying for goods and petrol at The Co-operative Food, this week.

Customers who used a credit or debit card to transact with the Co-op on 7 July were charged twice, the grocer has admitted.

The Co-op said: "We would like to apologise to all those affected, and reassure customers that refunds will be made directly in to their accounts within 24 hours.  We will also reimburse any customers who have incurred bank charges as a result of this error."

Geoff Barraclough from the retail technology and payments consultancy Barraclough & Co said that while the industry does not yet know the source of the Co-op's payment troubles, he would always recommend retailers using an outsourced payments service provider. "Outsourcers can still fail of course, but payments is a specialised domain that's best entrusted to expert suppliers."

He added: "Given the volume of transactions processed everyday, it's a testament to the strength and resilience of the UK's payment infrastructure that this kind of incident is so rare."

"Nevertheless, it's very embarrassing for the Co-Op and underlines the very public nature of payment-related IT failures."

George Scott, senior consultant at Conlumino, said these types of payment problems do happen and it is not unique to the Co-op.

"It could have been a lot worse – it could have been a big data breach," he said. "But this could well have been a human processing error, or issues relating to refitting their stores for contactless payments."

Scott said we may not find out what the cause of the problem is, but the grocer's speedy announcement and promise of a refund within 24-hours will do a lot to mitigate the negative sentiment felt by customers charged twice.