A spokeswoman for the company, which is a division of Halifax bank, said it was forced to take down its Internet banking site from Monday morning until Wednesday evening.
She said the problem was caused by "a rogue piece of coding" which had either been entered or triggered during a systems upgrade, making the site unusable under heavy volumes of traffic.
When asked why it had taken three days to locate the piece of rogue coding, fix the problem and bring the site back up, the spokeswoman pointed to the state of modern technology and likened the hunt for the offending code to slowly peeling away the layers of a large onion.
While admitting that the loss of a Web banking service was an inconvenience for customers, she said it "was not really that big a problem" for IF because its customers were still able to access a full banking service over the telephone. She denied that the confidentiality of customers' details had been compromised, and said the cause of the problem was still being investigated.
Analysts expressed surprise at the length of time the IF Web service was unavailable.
Daniel Mayo, lead analyst in the financial services group at Datamonitor, said the incident raised some serious questions about the company's testing procedures, its back-up policy and its business continuity plans, and should result in a thorough review. "This is a pretty serious incident for them," he said.
Mayo pointed out that although customers may have been able to carry out a full banking service over the phone, IF is fundamentally an online brand and customers might leave the bank as a result of the incident.
Gavin George, a partner at IT consultancy itim, said, "There is no doubt that the brand would be impacted by an outage of that length. It undermines the whole positioning of the service. It is a serious issue for them. It suggests that it is something more than just a bug or a piece of rogue coding."
IF is not the only online bank to suffer from downtime woes recently. In August, Computer Weekly revealed how up to half of Barclays Bank's customers were affected when a computer outage forced it to take down the systems supporting its branches, cashpoint machines and online accounts for an hour during peak demand.
Last year database problems prevented HSBC customers from accessing account information for more than 12 hours.