Taxpayers who use forms that must be revised as part of an AMT fix, but who are not subject to the AMT, also are affected by the Temporary Tax Relief Act of 2007. These include low- and middle-income Americans who claim valuable tax credits, such as the Dependent and Child Care Credit, education credits and the Retirement Saver's Credit.

"This delay has the potential to be a serious disruption for the 2008 tax season," said Tim Gokey, president of H&R Block Tax Services. According to Gokey, "H&R Block has been working with the IRS to minimize taxpayer burden as a result of the late legislation."

The IRS estimates it will begin processing returns using the revised forms 10 weeks after Congress approves new tax law. The House of Representatives passed the legislation on Nov. 9, and the Senate will take up the bill when it returns the week of Dec. 3. The IRS will accept electronically filed returns starting Jan. 11. Using the 10-week timeframe, legislation passed Dec. 21 would not permit the IRS to process affected returns until Feb. 29.

About 60 percent of H&R Block clients who claim the credits using the affected forms traditionally file by the end of February and therefore could experience hardships from a delayed refund needed to pay holiday bills or solve other immediate financial problems.

A ripple effect also could occur when the IRS begins processing returns. For example, returns filed later in the tax season that might otherwise be processed in a few days may experience a delay while earlier returns are being processed. And, depending on the size of any backlog when the IRS begins processing returns, the delay could last for the rest of the tax season and affect millions more taxpayers, according to H&R Block.