You have less than a month to file your self-assessment tax return online before the January 31 deadline.

About 10 per cent of forms will be submitted late, incurring a £100 fine. But accountants caution that there is another factor that should push you to file your return on time.

The Treasury is facing a large shortfall in the amount of tax that it is likely to collect. In the Pre-Budget Report, the Chancellor admitted that the economic downturn will reduce the Government's tax take sharply, leaving a hole to be filled.

Mike Warburton, of Grant Thornton, the accountant, says: “We have shrinking tax receipts but growing government expenditure. HMRC is under instruction to improve its tax collection techniques.”

So the Revenue will be analysing returns even more closely this year. While the majority of Britons pay tax through PAYE, the pay-as-you-earn scheme that deducts tax directly from wages, more than nine million workers - the self-employed, higher-rate taxpayers and those who earn income that is otherwise untaxed - are expected to complete a tax return each year.

The deadline for paper returns was October 31, so your only option now is to file online by the end of the month. The Revenue estimates that a record 3.8 million taxpayers will use its website to submit their returns.

In recent years computer systems have been used to flag up anomalies, analysing year-on-year patterns to your returns. For example, the Revenue will be looking for changes in capital expenditure. In previous years, a variation of 20 per cent year-on-year might have raised suspicion, but Angela Beech, of Blick Rothenberg, the accountant, cautions that a even a 10 per cent increase in expenses could now prompt a call from the Revenue, or even a tax investigation.

Ms Beech says: “In the past the onus was on you to self-assess your income. Now, however, it is more likely that the Revenue will demand proof.”

If the Revenue does query any details on your return, there are steps you can take to ensure that you are not dragged into a lengthy investigation. Ensure that you have all the paperwork to back up your return. For example, if you have made a charitable donation, find letters from the charity or bank statements confirming it. If you claim back tax for pension contributions, ensure that you have statements from your provider. If you cannot find them, order copies.

It is essential to keep all your receipts to hand, backing up your tally of expenses for the year. It is also better to respond promptly to the taxman if you receive a query. Ms Beech says: “The Revenue is more likely to think that you have nothing to hide if you do not delay.”

If you think that your earnings will be reduced substantially, you can apply to lower your interim payments. But you will need a good reason and evidence, Mr Warburton says. The relevant form, SA303, is available on the website and from your local tax office.

The Revenue claims that it has worked hard to simplify the tax return and the process of completing it online. But if you have any questions or are unsure, phone the helpline on 0845 9000444. It is open seven days a week from 8am until 8pm. The website, at www.hmrc.gov.uk, also has useful guides and details on what you can and cannot claim for.

Do not leave it until the last week of January because that is when the Revenue's servers come under the greatest strain. In previous years the system has crashed, incurring an enormous amount of frustration for those who waited until the last minute. Avoid the stress by starting the process this weekend.

If you are filing online for the first time, remember that you need a registration number, which is sent to you through the post. This could take up to a week to arrive.

Case study: Organisation is key

Jackie Cambridge handles the accounts for Cambridge Safety, the health and safety consultancy that she and her husband, Paul, run in Peterborough.

She has already taken care of the self-assessment tax return for this financial year, having completed the online form in September, well before this month's deadline.

“I have been doing these returns for ten years, so I'm pretty organised now,” the 41-year-old says. “It is much easier if you keep your paperwork in order for the entire year, rather than looking for it when you come to fill in your return. Find out what you can claim for and make sure that you keep all the receipts and invoices you may need.”

The couple set up the business in 1998, training students to be health and safety advisers or inspectors. The couple also work as practitioners themselves. The business has a turnover of £250,000 a year and employs three members of staff.

Mrs Cambridge says that taxpayers filling in the form for the first time this year should count themselves lucky, because the modern forms are much easier to navigate than older versions.