Those who have yet to file their online tax return with HMRC for the 2014/15 tax year are being urged to do so by the end of this month.
The additional £10 daily charge will be levied on taxpayers for 90 days if they failed to submit their return by the end of April 2016.
The legal filing deadline for online self-assessment is on 31 January. Those that fail to file their return face a fixed penalty fee of £100.
Anyone who still has a return outstanding at the end of July will have accumulated fines of £1000, including the additional £10 per day penalty.
Outstanding returns that are not filed to HMRC by 31 July 2016 will face a further penalty of at least £300.
Anthony Thomas, chairman of the Low Incomes Tax Reform Group, said:
“Importantly, the fines can be appealed against if you have a reasonable excuse for filing the return late. This might include situations such as prolonged ill-health, bereavement, or family breakdown – to give just a few examples.
“We would strongly urge anyone with an overdue return to submit it as soon as possible; and to do so online as a paper return for 2014/15 will already attract the maximum £1,000 penalty. You can then appeal against the fines by writing to HMRC to explain why the return has been filed late.”