The Revenue's self assessment web portal was running "very close to capacity" on the filing deadline of 31 January 2005, said today's National Audit Office report.
The NAO said HMRC recognises that it needs to ensure that its SA Online service is "fully available and accessible at all times".
The report described some of the problems encountered by taxpayers and advisers during the weekend of 29 and 30 January 2005.
"The service did not send out an instantaneous success or error message to around 80,000 people when they submitted their completed return," it said, adding that the problem was fixed and all the stored messages were sent out early on 31 January.
"HM Revenue and Customs recognised that people who had tried to file over that weekend and received an error message on 31 January may no...