update ING Direct has confirmed some customers' bank accounts were rendered inaccessible on Sunday, after a problem in the middleware layer of the bank's IT infrastructure caused an outage.

An ING spokesperson told ZDNet Australia that intermittent middleware problems throughout Sunday meant customers could have received an "access denied" error when trying to login to their account.

Customers attempting to access the site around 3.20pm on Sunday reported difficulty entering login details and accessing their account balance.

Although the spokesperson could not disclose the exact length of the outage, they said they believed it was "only a couple of minutes, here and there".

However, callers to ING's customer service line reported call centre staff claimed they were also unable to access the accounts, and that the bank's online systems would be down for several hours.

ING technology staff are currently conducting a root-cause analysis of the problem, according to the spokesperson, who was unable to name ING's middleware provider.