Agency officials said the failure was an "isolated and temporary" incident and that large numbers of people attempting to access the site at once were not to blame, The Times of London reported Thursday.

Late filers could be fined almost $200 and 5 percent interest penalties on owed tax. HM Revenue & Customs has yet to say if it will enforce the fines in spite of its Web-site problems, The Times said.

The returns of 5 million people who used the department to file their taxes last year were a month behind because of HM Revenue & Customs' "production issues."

If taxpayers don't complete payment on all due taxes before July 31 any fines could double.