Led by industry group Tax Executives International, companies had called on the Financial Accounting Standards Board to delay the new rule, which takes effect this year. But the FASB, meeting in its Norwalk, Conn., offices, rejected any postponement by a unanimous vote.

The rule, known as Fin 48, requires companies to examine and document their tax positions, evaluating whether those that aren't certain are more likely than not to withstand a challenge by the Internal Revenue Service. Companies will only be able to book a portion of the tax benefit from positions that meet this threshold. In the past, companies had great leeway in deciding what tax benefits may withstand scrutiny.

Previously taken tax benefits that don't stand up to the new standard will result in adjustments to companies' tax reserves as they first start applying the new standard in their first-quarter results. Companies will give a range of the likely impact in their annual reports for 2006, but won't actually begin applying the rule on an annual basis until next year.

Regulators, along with FASB, hope the more stringent approach will help make companies' tax practices more comparable for investors, while additional disclosure will give them more information about the types of tax positions companies are taking.

While FASB members expressed some sympathy for companies grappling with documentation requirements associated with the rule, as well as more difficult judgment calls that have to be made, they said this didn't warrant a delay. Board members noted that they had already twice delayed the effective date for the rules, which companies will have to begin following with issuance of their first-quarter reports, and that under existing rules companies should already be documenting uncertain tax positions.

FASB Chairman Robert Herz said the new rule was prompted by officials at the Securities and Exchange Commission who found that companies weren't taking a uniform approach to the way they booked potential benefits from tax positions.