NEW YORK (CNN/Money) - The federal government contends that General Motors' pension fund is $31 billion short of what it owes its work force and retirees, a figure strongly disputed by the automaker, which argues its plans have $2 billion more than necessary to cover anticipated benefits, according to a published report.

The New York Times reported that despite wide difference in the reading on the automaker's pension plans' strength, both estimates are within accepted accounting practices. The government's estimate is based on a snapshot of how current assets would cover promised benefits if the plans were terminated. The automaker's estimates assume the plans will continue to operate going forward.

"There is no reason to expect that G.M.'s pension plans will be terminated, and to assume otherwise is to unnecessarily alarm the many thousands of people who rely upon those plans for their retirement," GM spokesman Jerry Dubrowski told the paper.

The automaker has about 600,000 workers, retirees and surviving spouses expecting benefits from its two plans, one for hourly workers represented by the United Auto Workers union, and one for salaried staff, according to the paper. It has contributed more than $56 billion over the last 12 years to fund the pension plans, and by its estimates today it has total assets of $91 billion, and total benefits owed of $89 billion, according to the paper.

But the paper said that closely-held government data on the GM pension funds show that if it was terminated, its assets would fall $31 billion short of what it could cost to buy annuities to pay promised benefits to all the plans' participants.

The Pension Benefit Guaranty Corp. (PBGC), the federal agency that guarantees private-sector pension plans that pay a set benefit, made its own calculation at the end of June about the strength of the GM's plans and released the figure to the Times in response to a request under the Freedom of Information Act, the paper reported.

The estimate does not mean that GM must make a payment to cover the government's shortfall estimate or that it is facing any immediate cash crunch due to its plans, the paper reports.

But the estimate about the plan's shortfall on a termination basis could become more important as the Senate debates a broad pension bill that would require companies with junk-level credit ratings to start disclosing their pension values on a termination basis. GM's debt was downgraded to junk bond status in May by rating agency Standard & Poor's.

The legislation, which has bipartisan support as well as backing from the administration, is part of an effort to tighten the pension funding rules, in hopes of preventing more big failures like the one at United Airlines.

United continued to report its pension plans' strength on a continuing basis, even as its bankruptcy court proceedings dragged on more than two years. It reported a $6 billion shortfall as of the end of 2004, but when the PBGC finally took over the plans earlier this year, it recalculated them on a termination basis and found a total shortage of $10.2 billion, the largest funding gap of any plan ever assumed by the agency.

Generally a company must have filed for bankruptcy protection in order to have the PBGC take over its plans. While GM has struggled with losses from its core North American automotive operations, most analysts believe it is not in any imminent threat of filing for bankruptcy, especially since it has roughly $20 billion in cash on hand, according to the report.

For a look at whether bankruptcy is a realistic option for GM, click here.

For a look at the health of the nation's pension plans, click here.