If you sent your tax return back late, or paid your tax late, you'll have to pay a penalty. If you think you had a reasonable excuse for being late, you should contact HM Revenue & Customs (HMRC). If you thought you didn't need to send a Self Assessment return this year, you should get in touch too.
The deadline for sending your online tax return was midnight on Tuesday 31 January 2012. All tax returns received on 1 and 2 February 2012 will be treated by HMRC as though they were received by 31 January. Paper tax returns were due on or before 31 October 2011.
If you've missed the deadline you'll have to pay a penalty of £100. You should send your tax return online as soon as you can to avoid further penalties. Don't send a paper tax return as the penalties will be even higher.
The longer you delay, the more you'll have to pay. When your tax return is three months late, you'll have to pay a penalty for each additional day it is late. When it's six months late, you'll have to pay a further penalty and another final penalty when it's 12 months late. Together these can add up to a penalty of £1,600 or more.