NatWest's online banking service, natwest.com, was down for three days. Although customers could access the site they could not view their balances or complete any transactions.
The Web site of Intelligent Finance, a division of Halifax bank, was down for the first three days of last week after "a rogue piece of coding" resulted in the systems grinding to a halt.
Egg's site was down for an hour on Monday morning after what a spokeswoman called "an isolated technical failure".
The spokeswoman said the fault on Egg's systems was not due to a software upgrade, security problems or network failure.
Paul Gilmore, technical director of e-banking consultancy ASpace Solutions, warned that online banks were pushing product innovation at the expense of good customer service.
"By factoring more time into product testing and release plans, these software glitches will become less frequent," he said. "However, perhaps they also need to be more realistic about their service capabilities and acknowledge that 24x7 availability is not achievable."