Tax experts are calling for an investigation into why more people are failing to cope with the Inland Revenue's self assessment system.

The Institute of Chartered Accountants said four million tax returns were still outstanding, and this could result in a record multi-million-pound windfall of fines and surcharges for the Treasury.

In the last three years, the Institute claims self-assessment penalties have increased five-fold, to more than £50m.

Julia Penny, president of the Thames Valley Society of Chartered Accountants, said: "The number of people who send in returns late, or not at all, is increasing and we need to know why. The system is very complex, which may be the reason, but it's in everybody's interests that some attention is given to what's going wrong."

Taxpayers who do not use a professional adviser have to work out how much they owe, but Mrs Penny said they could easily be baffled by the tax calculation guide.

Those who miss the deadline will be fined £100, and may have to pay interest on outstanding tax at the rate of 6.5 per cent until the debt is settled.

In response to the growing problem of late filing, the Inland Revenue has introduced a system of cold-calling to remind taxpayers of the deadline.

An Inland Revenue spokesman said part of the problem was down to the increasing number of people completing self-assessment. But he said 90 per cent of forms were returned before the deadline.

He said: "We have introduced new services as part of our customer service to help people complete their returns correctly and on time.

"Taxpayers can also obtain free tax advice by contacting their own tax office. They have nine months to complete and submit their Tax Return before incurring a penalty."