WASHINGTON (AP) _ For the estimated 34 million taxpayers who have yet to file their income tax returns: The headaches will only get worse if they miss Thursday's deadline.

Although the Internal Revenue Service offers extensions, they can be quite costly. And the worst thing to do is not file anything by the stroke of midnight on April 15.

``The worst strategy, one that can add thousands of dollars to your tax bill, is to do nothing,'' said Lawrence Torella, an accountant and partner with Richard A. Eisner & Co. in New York.

Torella said a ``whole new generation'' of taxpayers with problems filing returns on time could be created by income from the booming stock market, an increase in the number of people who have stock options and windfalls from sales of small businesses. In addition, tax returns are more complex than ever.

The penalty for failing to file a tax return is 5 percent of the balance due per month, up to a maximum of 25 percent.

Taxpayers who aren't going to meet the filing deadline should send the IRS a request for extension until Aug. 16 using Form 4868, which is available on the Internet, IRS offices and some banks and post offices. The IRS automatically approves such requests.

But people seeking extensions should still pay an estimate of their taxes due, or at least as much as possible, by Thursday. Doing so will lessen the sting of interest _ now 8 percent on late payments _ and penalties of a half-percent per month.

The IRS will refund any amount overpaid, once the final return is submitted.

``Come in a little high,'' said IRS spokesman Don Roberts. ``Whatever you have left, you're going to pay interest on.''

Penalties for late tax payments drop to a quarter-percent per month if the taxpayer enters into an installment payment agreement with the IRS.

A request for such an installment plan, using Form 9465, can be attached to the tax return. The IRS almost always grants such requests as long as the taxpayer agrees to pay $25,000 or less within five years. There's a $43 fee to set up such a plan.

For reasons such as a death in the family or serious illness, the IRS sometimes waives late-payment penalties on returns filed on time. People should make such requests courteously and in writing.

One other note: The U.S. Tax Court has rejected claims from taxpayers who say accountants advised them to file returns late while collecting required documentation.

Through April 2, the IRS had received almost 70 million of the 126 million tax returns expected this year. Counting estimates for this week, IRS spokesman Roberts said it's likely that 34 million taxpayers, or about 26 percent, will submit their returns in the next few days.

Although refunds are running close to 20 percent ahead of last year and average $1,575, it's likely that people who owe taxes make up the bulk of the last-minute filers. And many people probably postponed doing taxes last week because of the twin Easter and Passover holidays.

``You really have only one good weekend in April to work on tax returns, instead of two,'' Roberts said.

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Last-minute forms from the IRS are available in several ways. The fastest is through the Internet at www.irs.ustreas.gov. Taxpayers also can call from a fax machine at 1-703-368-9694 and have a form faxed.