NEW YORK (CNNMoney.com) - New stricter pension accounting rules by private watchdog group the Financial Accounting Standards Board could cost billions for shareholders, according to HR company Towers Perrin.

78 of the Fortune 100 companies surveyed offered defined post-retirement benefit plans. These companies could see a $180 billion -- or 9.3 percent -- reduction in shareholders' equity.

The stricter rules are expected to be proposed by the FASB in March.

The FASB plan has two components: Cleaning up year-end balance sheets and forcing income statements to more accurately represent changes in the fair value of assets and liabilities from year to year.

Phase I would likely effect year-end 2006 financial reporting, while Phase II would take much longer.

The study estimates that -- had the rules been in effect for 2004 -- companies with pensions would have been forced to recognize $331 billion of liability on their balance sheets for the year, instead of the $62 billion that was actually recorded.

"As our analysis shows, these accounting changes are certain to have a dramatic impact on many companies' financial statements in the years ahead," said Bill Gulliver of Towers Perrin.

"However, it is uncertain exactly how the market will react because most of the information expected to find its way onto the balance sheet is already disclosed in financial statement footnotes. Consequently, many analysts are already using that information to adjust the balance sheet today."

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Germany, facing massive pressure from pensions, will raise its retirement age from 65 to 67 earlier than planned. Full story here.

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