Taxpayers face paying out hundreds of millions of pounds in penalties over filing their tax-returns late.
The 31 January tax deadline for self-assessment forms is only weeks away, with those who file their returns after the deadline facing stiff penalties.
Research by unbiased.co.uk found that Britons gifted the taxman an estimated £368 million last year because of late returns and other miscalculations.
It was found that some £116 million in penalties were dished out because returns were received after the 31 January online deadline.
Another £16 million was paid in surcharges for late payment of tax from previous years.
But it was miscalculations made on tax forms that hit people in the pockets the most, with fines totalling £236 million finding their way to the taxman because of such errors.
Failure to return forms to Her Majesty's Revenue & Customs (HMRC) before the fast-approaching online 31 January deadline will incur an automatic £100 late payment charge.
If the form is not returned by three months after the deadline, there is a penalty of £10 for each following day, up to a 90 day maximum of £900 plus the £100 penalty.
If the form is not returned by 29 July 2012 there will be a further penalty of either £300 or 5% of the tax due, whichever is the higher. There are also strict penalties for late payment of tax.
"The rules of self-assessment forms remain unforgiving for taxpayers who return their forms incorrectly or who fail to meet the deadline," said Karen Barrett, chief executive of the website.
"The monetary penalties are hefty and in most cases additional surcharges are often imposed. In order to avoid this unnecessary waste we are urging consumers to take 'tax action' and act now."
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