
The company's card processing system is used by a large number of internet stores to process payments for online transactions.
A Barclays spokesman said the ePDQ system encountered a major hardware failure at about 3pm yesterday.
He confirmed that the systems are still down, and that engineers are working on the problem. Barclays hopes to have it resolved in the next few hours.
Phil Atkinson, vice president of sales at Eleventeenth Ltd, a preferred partner of Barclays' ePDQ system,said that the failure has caused major problems for its clients as well as for Barclays itself.
The Barclays spokesman said: "We deeply apologise to our merchants that this has affected, and will resume full service as quickly as possible."