About 8.5 million higher-rate and other taxpayers are due to submit their 1991-92 tax returns to the Revenue by 31 October.
But many of them will miss the deadline because they are unaware of it or because they do not appreciate its significance.
Loughlin Hickey, a tax partner at the accountants KPMG Peat Marwick, believes that many wives, for example, do not know about the new obligation on them to send in their own tax returns under the system of independent taxation introduced last year.
He believes that the Revenue is probably still tracking down people who should have made a return last year but did not do so.
He said: 'The Revenue believe they have issued a very clear warning to everyone that they are treating 31 October seriously.'
The Inland Revenue says it is unable to assess what tax liability is due in time to meet the statutory payment dates if returns are sent in any later.
Nigel Edwards, a partner with the chartered accountants Butterworth Jones, said: 'It is the duty of individuals to advise the Inland Revenue if they have a source of taxable income. If there is any doubt in your mind about whether some income is going to result in a tax bill, you should tell the Revenue anyway and let them decide.
'They do have power to obtain information, and cross-checking of information is not uncommon. For example, one person's rent payment is somebody else's income.'
Return forms are sent out to taxpayers in April. Theoretically they are due to be completed within 30 days but the Revenue has extended the deadline to the end of October.
All higher-rate taxpayers and anyone else who receives income that is not fully taxed at source is obliged to fill out a tax return - even if they have not automatically been sent one by the Revenue.
Higher-rate taxpayers, people who owe tax on bank deposit accounts, for example, and those who have a capital gains liability are due to pay the tax on 1 December.
The country's 3.5 million self-employed people who pay Schedule D tax have to make their first payment on 1 January. If their tax returns are late this can lead to late assessments and payments, which may mean fines and interest charges.
Penalties and interest are incurred for failing to pay tax when it is due, and also for failing to fill out a tax form, for submitting an incorrect return either fraudulently or negligently, and for submitting incorrect accounts or claiming incorrect allowances.
The interest rate charged by the Inland Revenue on tax paid late is currently 9.25 per cent. This is payable gross and is not deductable from profits or income.Reuse content