Some Barclays customers have been unable to log into their online banking accounts due to high traffic volumes accessing the website.
Customers have experienced slow responses from Barclays Online over the past two days as a high volume of people logged onto the site.
A spokesman said a small minority of customers were affected.
In a statement, the company said, "We apologise to any customers who may be experiencing slow responses from Barclays Online Banking today. We are seeing higher than normal volumes of traffic but the service remains available and we are working to stabilise it as soon as possible."
The spokesman added that volumes of traffic had increased due to an HM Revenue & Customs (HMRC) income tax payment deadline on Sunday.
"It's only a very small percentage of customers who are experiencing problems. For the overwhelming majority of people the service is fully available," the spokesman said.
He confirmed Barclays has considered taking the service "off-line" to "refresh its infrastructure". However, due to the small number of customers affected, Barclays plans to rely on "standard overnight refreshes" to fix the problem.
The company said the number of customers experiencing problems was decreasing. The spokesman predicts Friday will be the busiest day due to income tax traffic.
Customers unable to log into accounts vented frustrations on social media platforms, such as Twitter, complaining the online banking service has been unavailable all day.
Some small businesses complained at being unable to pay suppliers and staff members.
The investment division of Barclays Bank recently deployed network infrastructure automation firm Infoblox's IP address management platform across its global network to reduce network downtime.