HM Revenue & Customs has already begun sending out penalty notices telling people they have been fined £100, despite the fact that online tax returns are not due until 31 January.
It's the latest in a series of blunders that saw Britain's top taxman, David Hartnett, earlier this month announce his resignation next year. His time as head of HMRC has seen computer cock-ups that have left millions of workers with the wrong tax codes and facing hefty unexpected tax demands.
Mr Hartnett has also faced criticism for the deals the Revenue has struck under his leadership with firms such as Goldman Sachs and Vodafone, which are alleged to have cut their tax bills by billions of pounds.
Angela Beech, a senior tax partner at the London accountancy firm Blick Rothenberg, said: "The Revenue has got it wrong again and is causing unnecessary distress during the festive season. We have seen several demands and while the deadline for paper returns is passed, people have until 31 January to file online."
Ms Beech said that in one instance, a client who had been told he did not need to complete a return has already been fined £100 for not sending any return. "Not only that, but for every day the return is late he will be fined £10. People are being asked to pay their fine within 30 days and are being told that if they haven't paid by then, penalties of £10 per day can be charged for the next 90 days, making a whopping £1,000 fine for a tax return you didn't have to file."
She blamed HMRC computers which capture the wrong information about the filing of paper returns. "This error by the Revenue has resulted in erroneous and threatening demands to be issued to taxpayers across the country."