As the countdown to the income tax filing deadline entered its final stretch Monday, an estimated 26 million taxpayers had still not filed or finished their forms, the IRS said.

In most parts of the country, Uncle Sam must be reckoned with by midnight Monday. Patriot's Day, a regional holiday in New England, gives residents in Massachusetts, Maine, New Hampshire, Vermont, Connecticut and Rhode Island, as well as northern New York state, until midnight Tuesday.

Despite the time crunch — and the considerable penalties and consequences that can face those who fail to file or file late —the IRS said procrastinators should not sacrifice accuracy for speed. The tax agency advises taxpayers to double-check their math, make sure to use the correct amount from the tax table, and be especially diligent in making sure Social Security numbers, including those for dependents, are listed correctly.

The looming deadline doesn't just present a crunch for America's lazier taxpayers. IRS spokesman Don Roberts said the agency would received an estimated 37 million tax returns in the final two weeks before the deadline — 11 million last week and 26 million expected to be postmarked or electronically filed on Monday. About 130 million individual returns in all are expected this year.

Additionally, the post office is putting in overtime. Because the IRS accepts a postmark as proof the return was filed on time, many post offices around the country will be open late to handle returns.

For those taxpayers who just can't file by deadline or who can't pay what they owe by Monday night, the IRS offers a variety of filing extension and installment payment programs.

Taxpayers can request an automatic four-month extension, but they should pay their estimated taxes on time. Otherwise, the IRS will assess a late-payment penalty and interest currently running at 8 percent.

Extensions can be obtained by phone by calling (888) 796-1074 — some information from the 1999 return is needed to verify identity — or with Form 4868.

Those who owe money should mail a check or money order made out to "United States Treasury," rather than the IRS, and include a Social Security number, the year and the type of form filed. For most people this would be "2000 Form 1040."

For people facing a big tax bill they cannot pay all at once, the IRS offers several options. One is credit-card payment through one of the two toll-free numbers: (888) ALL-TAXX or (800) 2PAYTAX.

The call is free, but the companies that run the services for the IRS collect a convenience fee for the transaction. The IRS points out that it gets none of this money, nor does it collect and store credit-card numbers.

The IRS also will most likely approve a request for an installment payment plan if the taxpayer owes less than $25,000 and can pay within a five-year period. To obtain such a plan, attach Form 9465 to the front of the return; there is a $43 fee to set up an installment plan, which carries 8 percent interest and a penalty of 0.25 percent per month once it is approved by the IRS.

The IRS also has a program for people with big debts they cannot possibly pay. Known as the offer-in-compromise program, it allows taxpayers to negotiate a lower settlement of their tax debt.

The program has become so popular that the IRS has a huge backlog. The General Accounting Office, which is the investigative arm of Congress, estimated that the number of offers grew from 32,300 in 1997 to 87,500 in 2000.

"That is actually one of our problem areas," said IRS Commissioner Charles Rossotti. "It's going to take some time to get the backlog down."

— The Associated Press contributed to this report