The deadline for those want their liability for the 2001-02 tax year worked out for them passes on 30 September.

After that, tax payers will have to employ an accountant or work out for themselves how much they owe the taxman.

Self-assessment tax payers that miss the September deadline have to return their forms with full payment to cover what they owe by 31 January.

Late penalties

Missing this deadline carries an automatic £100 fine, with interest charged on any overdue tax.

Main groups self-assessment applies to

Self-employed Trustees or personal representatives Business partners Company directors, employees and pensioners with complex tax affairs

If the tax payment is still not made by 28 February, an additional surcharge of 5% on the outstanding balance is imposed.

Self-assessment forms received by the Inland Revenue after next January will be open to a full investigation by the taxman for up to 15 months.

The normal enquiry period for forms filed between September and the end of January is only 12 months.

Millions miss deadline

Self-assessment affects 9.5m tax payers with more complicated tax affairs.

The Inland Revenue told BBC News Online that 4.8m missed the September deadline last year.

What is more, 800,000 failed to file a return by 31 January and were fined.

Chas-Roy Chowdury, head of taxation at the Association of Chartered Certified Accountants (ACCA), believes self-assessment tax payers are best to get their skates-on and make the September deadline rather than waiting.

"The sooner the forms are sent in, the sooner tax payers will discover whether they are entitled to tax rebates," he said.

Tax payers can fill out self-assessment forms online.

This can be done by going to the Inland Revenue website at www.inlandrevenue.gov.uk to file their tax return.