Barclays has confirmed some customers are unable to log into online bank accounts and are experiencing slow response times.
Customers encountered extended response times using Barclays' internet banking service for the past two days. Some customers are now unable to log in to online accounts at all.
A spokesman confirmed the problems are affecting a segment of customers: "Some can log in to online banking, some are experiencing slow response times while others are unable to log in at all."
The spokesman added that the root cause of the problem is yet to be identified.
"We advise customers to try again later or use the telephony services until the service is back to normal. We hope to have the service back-up as soon as possible," he said.
Customers complained on social media platforms, such as Twitter, about being unable to log in, being kicked out and unable to pay bills on pay day.
One user tweeted, "Barclays has the most frustrating and impossible online banking service imaginable. Fix it - other banks manage to at least let me log in."
Another user said, "Why won't Barclays online banking work? It keeps saying I have been logged in for 10 minutes when it's been about 10 seconds."
Barclays' customers experienced problems with the online banking service in July 2011 after increased website traffic due to a HM Revenue & Customs (HMRC) income tax payment deadline.
The bank was forced to take the service off-line to refresh its infrastructure.
Barclays posted an 18% increase in pre-tax profits for the three months ending September 30, compared to the previous year.