Egg credit card customers have been unable to use certain debit cards to make repayments through the group's website for the past six weeks, it emerged today.
People with NatWest or Royal Bank of Scotland debit cards have been unable to use the Egg website to make payments to their accounts as the site has not recognised their cards.
The problem is thought to stem from the website not recognising the card's bank identification number (Bin) - the first six digits that appear across the front of the card.
NatWest and RBS are currently in the process of migrating their 13 million current account customers from Maestro to Visa debit cards.
The problem with Egg is thought to have occurred because its systems have not been updated to recognise the new numbers.
Egg said it has a contract with Experian Payments to receive updates on new Bins and it is not able to update its systems manually.
An Egg spokeswoman said: "As soon as we were made aware of this issue we have been working with the third party who provided the data to rectify this problem as quickly as possible. We expect it to be resolved early next week.
"As the majority of Egg customers pay by direct debit this only affects a very small number of customers wishing to make a top-up payment.
"Customers who have called us have been informed of alternative payment methods and there has been no impact on their account. We apologise for the inconvenience this has caused."
An RBS spokeswoman said: "In line with scheme rules, we provided all the necessary information to Visa to enable them to communicate changes to bank identification numbers ranges for our new Visa debit cards. Visa communicated these changes to acquirers in good time.
"It is the responsibility of the acquirer to inform their merchants of these changes, however, we are keen to resolve this problem for our customers as soon as possible, and we are working closely with Egg and all other parties to do so."
She added that it was an isolated issue, and customers could still pay their Egg credit card bill through RBS's online and telephone banking service or through their local branch.