Customers are complaining on Twitter that the website isn't letting them log in or access their accounts.
A spokesman for Barclays said on the bank's Twitter account that it is "aware of a known issue and we're currently working on a fix to resolve this".
People who first reported problems at about 12pm today said that the website had been asking them for four digits instead of the usual five when they try to log in, which didn't let them access their accounts.
Now online banking appears to have crashed completely for many customers, according to reports on Twitter and outage tracking website Down Detector.
Several are receiving an error message which tells them the website is "experiencing technical difficulties".