Tax officials are sending nearly one million penalties to taxpayers who failed to file self-assessment returns by the end of January, Sky News has learned.
By February 20, HM Revenue and Customs (HMRC) will have completed a mail-out of £100 fixed fines to around 850,000 taxpayers.
The letters are being sent to those registered for self-assessment (SA) but who failed to file personal 2011-12 tax year returns online by January 31.
The penalty payments will boost HMRC coffers by around £85m. The late filing figure is down on the penalty number for the 2010-11 tax year.
But the final figure is expected to grow significantly as those who fail to pay the penalty and file returns by the three-month cut-off incur additional daily fines .
Around 60,000 late tax returns have already been lodged between February 1 and 15 - but those taxpayers are still liable to pay the £100 penalty.
An HMRC spokesman told Sky News: "Anyone who hasn't yet sent their 2011-12 tax return to HMRC will have already incurred a £100 late-filing penalty.
"Non-filers have to file online now to avoid further penalties or contact us to ask to be taken out of self-assessment, and provided they meet the criteria, we will take them out of SA and cull any penalties incurred."
The Tax Office expects to have finished processing all 850,000 penalty letters by February 20.
Most of those hit by the late filing penalty are expected to receive their letters within seven days.
HMRC has so far received approximately 9.76 million returns, both paper and online, for the 2011-12 tax year.
In the previous tax year, 80.9% of the 10.5 million SA taxpayers filed on time for the January cut-off.
Taxpayers who fail to file returns after three months are hit with a daily £10 fine up to a maximum of £900, along with the initial £100 fine.
Those who allow the filing delay to extend beyond six months are handed another £300 fine or 5% of the tax due, whichever is higher.
HMRC is then entitled to give those who fail to file within 12 months a tax demand up to 100% of the tax due instead.
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