Users of the government-backed savings organisation National Savings & Investments (NS&I) have been struggling to access its Atos-managed website since yesterday afternoon.
According to a spokesperson for the organisation, the problems are "intermittent", with some customers being able to log on to the site and use the online services, while the website is failing to load properly for other users.
"We are currently experiencing intermittent difficulties with the website and we are working to resolve those issues as quickly as possible.
"We are very sorry for any inconvenience caused to customers," a NS&I spokesperson said.
The cause of the problem is as yet unknown, and the spokesperson said that technicians are currently working on resolving the issues.
Customers finding it difficult to access the website are being urged to carry out transactions over the phone instead by calling 0500 007 007.
NS&I has outsourced its back office operations to Siemens IT Solutions and Services, (now Atos), since 1999. While Atos are currently in charge of managing the organisation's IT and operations, last November, the organisation put out a routine tender for an IT outsourcing deal that could be worth up to £2 billion.