Barclaycard's integration with ex-Morgan Stanley firm Goldfish has caused severe disruption to customers trying to access their online accounts.

Some customers have been locked out of their accounts for more than two weeks, according to a Barclaycard call centre agent. This has driven users to register their frustration with the service in a web forum on MoneySavingExpert.com.

A Barclaycard spokesman claimed that the site's capacity was increased by 50 per cent last Friday.

"There is no intention to increase the site's capacity more than that," he said. "As customer demand reduces, more users should be able to access the service again."

The spokesman explained that Barclaycard had anticipated an increase in customers using the online service but had not expected the demand to be so huge. He insisted that there had not been a lack of forward thinking.

Barclaycard's call centre agents are informing customers that they will be notified in writing when services return to normal, although the Barclaycard spokesman said that this was not the case.

A spokesman for the Financial Ombudsman Service advised customers to make an official complaint with Barclaycard.

"This triggers a formal legal process and the bank has to respond in eight weeks to address it," he explained, adding that if the bank does not respond within two months, customers can then complain to the ombudsman.

To make an official complaint to Barclaycard, customers need to call a 0844 number that charges a minimum of 8p a minute for BT customers.

The Barclaycard web site's log-in page currently holds a service status update which reads: 'We are currently experiencing high demand, which may affect the speed of service. We apologise for any inconvenience this may cause.'

The news follows the Bank of America web site downtime in September 2008 which lasted for nearly five hours.