System problems occurred following routine maintenance over the weekend. They affected operations in the North of England between 11am and noon on Monday, said a spokeswoman for the company.
The problems emerged when the systems were brought back up on Monday morning. "Not all parts were running as smoothly as we would have wanted," she said. She refused to be more specific.
Barclays has two main computer systems - one for the north of the country and the other for the south. "Theoretically, it would have affected half of our customers," admitted Barclay's spokeswoman.
For a major bank such as Barclays, an hour of unscheduled downtime during the busy lunchtime period on a Monday is not a small matter. "There will certainly need to be a review of processes at the bank, especially the contingency planning and reporting processes," said Daniel Mayo, lead analyst in the Financial Services Technology Department at analyst firm Datamonitor.
Mayo said Barclays will want to find out if the outage could have been avoided and ensure that it is not repeated, as well as discovering what went wrong and why. The bank's IT department will probably have to assume responsibility for the situation, he added.
Graham Titterington, senior analyst at Ovum, said it was likely that the bank's systems could not cope as the volume of traffic increased on Monday morning following the upgrade. "It was probably a case of inadequate performance or load-testing," he said.