Anyone whose Self Assessment return is more than three months late will now be charged a further £10 penalty for each day it remains outstanding, up to a maximum of 90 days.
This is on top of the £100 late-filing penalty they have already received.
This means people who file their 2010/11 return online on or after 1 May will be liable to daily penalties, as the deadline for these was 31 January.
Daily penalties for paper returns began on 1 February, as paper returns were due by the earlier deadline of 31 October.
Looking ahead, further penalties of at least £300 (or 5 per cent of the tax due, if that is more) will be issued for returns that are 6 and 12 months late.
Anyone who's received a late-filing penalty, hasn't yet sent a return, but thinks they don't need to be in Self Assessment, should call HMRC on 0845 900 0444. If HMRC agrees, the return and any penalty will be cancelled. Visit www.hmrc.gov.uk/latetaxreturn for further information.
HMRC's Stephen Banyard said: "We want the returns and not penalties. So, if you haven't sent us your 2010/11 return, you need to do one of two things urgently - either send it online by 30 April, or call us if you think you shouldn't have to complete one."