Users attempting to file their returns were met with the message: “We are sorry but the service is currently unavailable ... We apologise for any inconvenience that this may cause.”
The site provides a link which promises further information, but only warns of future upgrade work due on 13 and 14 February.
A spokesperson told Computerworld UK that the site had gone down "some time this morning". HMRC was investigating the problem and was "working to restore the service as soon as possible", the spokesperson said, adding that individuals concerned about filing their returns on time could still hand in paper forms at a local tax office.
The news marks the second HMRC website problem in a week, after it admitted it had barred MPs and celebrities from using the service, insisting they should file their tax returns by post to ensure security.