Millions of taxpayers could find their refunds delayed next tax season due to Congress’s late action earlier this week to fix a glitch in the federal tax law.

The IRS said today that it had started to update its computer system to accommodate changes related to the Alternative Minimum Tax, passed this week by Congress and expected to be quickly approved by President Bush. But even with those efforts, the late start could delay tax-return processing—and refunds—for an estimated 15.5 million taxpayers.

“There could be delays for certain taxpayers, but we’re working to minimize the impact,” said Nancy Mathis, an IRS spokesperson.

Mathis said the IRS could not yet say exactly how many—or who—would be affected. However, based on earlier estimates by IRS Acting Commissioner Linda Stiff and the independent IRS Oversight Board, some 15.5 million returns—and some $39 million refunds—could be delayed.

The agency said it would be working over the next week to finalize its processes. Taxpayers should check the IRS’s Web site for updated information.

On Wednesday, Congress voted to protect some 21 million taxpayers from the AMT, a 1969 provision originally meant to ensure that the super-rich paid their fair share of taxes. Congress’s “patch”—an annual ritual that ran very late this year—insulates all but about 4 million taxpayers from using an alternative method to figure their federal income tax. The AMT, which disallows numerous tax breaks, raised affected households’ average tax burden by about $2,000 last year.

Yesterday’s patch raises the minimum adjusted gross income subject to the tax to $44,350 for single filers and $66,250 for married couples filing jointly.

In theory, people who aren’t AMT candidates, but who claim certain credits factored into figuring the AMT, could also experience delays. Affected credits include those for mortgage interest, education, child and dependent care, residential energy, retirement savings, and adoption.—Tobie Stanger