Her Majesty's Revenue and Customs (HMRC) expects to issue some 850,000 penalties for late tax returns over the next fortnight, 550,000 fewer than the same time last year.
Letters containing the £100 late-filing penalties are being sent to people who failed to send their 2010/11 Self Assessment returns to HMRC on time.
Anyone who still hasn't sent their return to HMRC should do so now or risk further penalties.
For example, anyone whose return is more than three months late will be charged an additional £10 penalty for each day it remains outstanding, up to a maximum of 90 days.
Although the deadline for receiving online returns was 31 January, this year HMRC is not issuing penalties to people who sent their 2010/11 return online on 1 or 2 February, following strike action at HMRC's call centres.
People who get a late-filing penalty can appeal against it if they think they have a reasonable excuse for not sending back their tax return in time.
The new penalties for late Self Assessment returns are:
• an initial £100 fixed penalty, which will now apply even if there is no tax to pay, or if the tax due is paid on time;
• after three months, additional daily penalties of £10 per day, up to a maximum of £900;
• after six months, a further penalty of 5% of the tax due or £300, whichever is greater; and
• after 12 months, another 5% or £300 charge, whichever is greater.
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