The retail giant's financial services business is in the final stages of moving operations over to its own systems and infrastructure based on the Fiserv Signature platform.

Over the weekend it transferred savings and loans products in-house from its old joint venture with the Royal Bank of Scotland, which it bought out in 2008.

This left some customers unable to get to their accounts online but on Monday Tesco claimed the problem had been fixed.

However, it now concedes users are still having problems, posting a message on the front page of its Web site to "apologise unreservedly".

The online problem has also led to a surge in calls to its contact centre, which in turn has left it unable to answer all queries "as quickly as you would expect".

"We have significantly increased the number of call centre staff available to help customers access their accounts. We are working hard to resolve any remaining issues that our customers have as quickly as possible," says the message.