By Zoe Kleinman
Technology Reporter, BBC News
Egg says its website should start accepting Natwest Visa Debit soon.
A database glitch has left NatWest customers with Visa Debit cards unable to make payments to Egg credit card via Egg's website for more than two months.
Egg, owned by Citibank, says that the fault is being investigated and should be fixed within the next few days.
The problem is that the Egg website does not recognise Natwest Visa Debit cards as being legitimate cards.
The first six digits of the long number on any card are known as the Bank Identification Number (BIN).
That number contains information about the bank issuer and card type.
Websites which process payments are programmed to search BIN databases for the number of the card that a buyer types in.
If the BIN number is not recognised then the card is declined.
Banks and building societies should receive regular updates of changing BIN numbers as new cards are introduced and old ones become obsolete.
Egg has a contract with Experian Payments to receive updates via a service called Bank Wizard.
Experian told BBC News that card issuers were responsible for supplying the updates to them and that they had been in touch with NatWest to resolve the issue.
"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," said a spokesperson from Egg.
"We expect it to be resolved by early next week."
According to Egg, the majority of its customers pay their bills by direct debit and have not been affected.
NatWest is in the process of phasing out its Maestro cards and migrating all 13 million of its current account customers over to Visa Debit.
"We are aware of this problem and raised it with Egg at the outset," said a spokesperson from NatWest. "We are now waiting for Egg to resolve the issue and update their systems."
Egg/Natwest customers affected can pay their bill via Natwest.
"In theory it should be relatively straightforward to make a change," said Akif Khan, head of client and technical services at payment management company Cybersource.
"All that needs updating is the underlying BIN table. You're not changing the code.
"It would be unusual for a financial institution not to get regular BIN number updates."