If you pay tax through self-assessment the deadline for online returns for the 2012-13 tax year is midnight on 31 January.
Miss the deadline and you'll face a fine. Fail to pay all your tax by the end of the month for the 2012-13 financial year and you will probably be slapped with a penalty charge.
Charges quickly mount up. If you're one day late you'll get fined £100, even if you have no tax to pay or have paid the tax you owe.
Three months late and there'll be an additional £10 for each day – up to a 90-day maximum of £900.
If you leave it six months you'll be charged a further £300 or 5 per cent of the tax due, whichever is the higher. A year on and there's another £300 penalty or 5 per cent of the tax owed.
In serious cases HM Revenue and Customs uses the rules to demand even more punitive penalties. It can actually demand that you pay up to 100 per cent of the tax due and in extreme cases the penalties can climb even higher.
In short, don't delay filing!