Although everyone who missed the January 31 deadline will automatically incur a Pounds 100 penalty, the amount of interest and other penalties facing individual taxpayers depends upon when they finally submit their return and pay some tax.
It is estimated that about one million of the nine million taxpayers who were sent self-assessment forms failed to send them in by last Saturday.
From February 1, the Inland Revenue started charging interest at 9.5 per cent.
On February 28, late payers will also incur a 5 per cent surcharge, which will double to 10 per cent from July 31.
Taxpayers who have not sent back their 1996-97 return by July 31 will incur a further Pounds 100 penalty. "People risk clocking up some huge charges, " says Maurice Fitzpatrick, a tax partner at accountants Chantrey Vellacott.
Most teachers who are taxed at source have nothing to worry about. Staff who are likely to have been sent a self-assessment return include peripatetic teachers with variable pay packets, teachers with additional income, and heads and deputies.
Many supply teachers registered with an agency may not have received a form because they are taxed at source.
However, Martin Benson of Coopers and Lybrand says some miss out on their personal tax allowance because they are always taxed at the basic rate.
To claim a rebate, supply teachers should contact their tax office and ask for a self-assessment form.