Her Majesty's Revenue & Customs is warning that taxpayers who submit their self-assessment forms late could be fined as much as £1,600 if they continue to withhold the information.
4 million taxpayers, who still have to send in their online 2010-11 forms by 31st January, have been sent reminders.
Tax officers are concerned they may not have realised that a much more draconian system of fines applies this time around.
To add to the longstanding fixed penalty of £100 for late filing, fines of £10 a day kick in after three months, up to a maximum of £900.
A further £300 can be levied after 6 months and another £300 after twelve months, adding up to a potential £1,600 in fines.
That's a high price for taxpayers who can't be bothered to send in a form or hold back deliberately.
When the Revenue introduced the new system of fines last year, it said it took a disproportionate amount of time and effort to deal with a hard-core of late payers.