The bank claims an intermittent fault in the switch brought down the entire secure customer service area of its Web site, affecting up to 1.33 million customers.
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The problem was initially said to be in back-end hardware and first began to affect customer log-ins last Monday. New sign-ups and the site's home page were unaffected.
Egg claimed to have implemented a fix by Thursday evening, but customer access problems were still being reported over the weekend and the site remained slow on Monday. Egg blamed this on "unprecedented demand".
It took Egg until the end of last week to post a statement on its site, apologising "unreservedly" to all customers who had experienced problems.
Graham Fisher, IT analyst at Bloor Research, criticised Egg for the delay in addressing the fault. He said he would have expected Egg to solve the problems far more quickly, especially as its business is based online. More importantly, he suspected a security breach may have been at issue and expressed surprise that Egg had taken so long to post a statement on its site.
"Companies that take this approach of not being upfront and honest are only damaging themselves," he said, adding, "Some people will be worried (by this) because privacy and security issues have a high profile at the moment."