The Internal Revenue Service says up to 100 million taxpayers - about two-thirds of all filers - won't be able to file their 2012 tax returns until late March if Congress doesn't adjust the alternative minimum tax by the end of the year.
Congress routinely adjusts the AMT to spare millions of middle-income taxpayers from steep tax increases. But the fix for 2012 is caught up in negotiations over the fiscal cliff.
Without a fix, the tax would hit an additional 28 million taxpayers. Others would face filing delays because the annual fix affects their tax credits.
The IRS assumed Congress would adjust the tax when it set up its computer systems. The agency says it would take until late March at the earliest to re-work its computers.