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NEW YORK (CNN/Money) - As if you don't have enough to worry about with 401(k) and Social Security benefits, here comes a new retirement woe: traditional pension plans may be in the worst shape of all.

The problems were highlighted after bankrupt United Airlines reached a deal this month to dump its underfunded pension plans on the federal agency that guarantees workers benefits. The UAL plans are estimated to be $9.8 billion short of the assets they need to pay promised benefits -- but they are hardly alone.

The Pension Benefit Guaranty Corp., the government agency that will now pay benefits to about 120,000 current and former United employees, estimates that traditional "defined-benefit" plans like the one at United are about $450 billion short of the assets they need to make promised payments.

Human resources consultant Watson Wyatt Worldwide says 614 of the companies in the Fortune 1000 firms offer traditional plans. Of those, it estimates 71 percent -- or about 435 -- have assets covering less than 90 percent of promised benefits. About half have assets that would cover 80 percent or less of what the plans are supposed to pay.

The pension problem has caused PBGC itself to be underfunded. After assuming the UAL plans, it will owe about $62.3 billion in promised benefits to more than 1.1 million people, including more than 500,000 current retirees, but its assets are only $39 billion. It has proposed raising funds by hiking premiums that companies with the traditional pension plans pay.

The Bush administration has proposed new rules to force businesses to close the gaps in many pension funds. But critics say some of the changes could unfairly hurt companies with basically healthy funds, perhaps driving some of them to stop offering traditional pension plans to employees.

Poor stock performance since 2000 is obviously part of the problem. So is the drop in interest rates, which affect assumptions about future returns.

But another part of the problem is that federal rules governing pension fund contributions give companies so much flexibility that only about 20 percent make the maximum contribution needed to have a fully funded plan in any year.

Companies even face a tax penalty if they contribute to a pension plan that has more assets than needed. That means when business is going well and the stock market and other investments are providing good returns, a company with a strong pension fund must stop making contributions, raise promised benefits, or face a tax hit.

Another part of the problem is kind of a reverse Darwinism.

Only a fully funded traditional pension plan can be terminated. So the thousands of companies that converted their old plans to 401(k)s over the last 20 years were the ones with healthier plans. What was left: a pool of pension plans that were in much worse shape.

The PBGC guarantees about 31,000 ongoing pension plans, down from about 112,000 in 1985.

Most companies with traditional pensions, even underfunded ones, are financially successful. But employers facing stiff competition from lower-cost upstarts without traditional pension plans often find their plans in trouble.

"A large amount of the liability is in airlines and steel," said Julia Coronado, senior research analyst at Watson Wyatt. "Autos are next in line."

Despite its recent woes, General Motors Corp.'s (Research) pensions plans were actually overfunded at the end of 2004, while assets in the Ford Motor Co. (Research) plans were enough to cover about 92 percent of promised benefits. The pressing retirement cost problems at those companies involve health care coverage for retirees, not guaranteed by any government agency.

But at some other old-line companies ran into financial problems, they were too cash-strapped to make the larger contributions necessary to fully fund the plans.

At the same time, some of those companies boosted promised retirement benefits to reach contracts with union workers, rather than raising wages, or even in return for short-term pay cuts.

Another problem is the one faced by Social Security -- as pension plans and Americans age, the ratio between active employees and retirees shrinks. In 1985, there were nearly four active employees for each retiree being paid benefits by a traditional pension plan. Today, it's closer to two to one.

The Bush administration wants employers with the plans to pay $30 a year for every current or former employee eligible for benefits, up from $19, a move that would increase total premiums to about $1.6 billion a year from about $1 billion currently.

The administration's more significant proposal would require all underfunded plans -- not just those with assets to pay 80 percent or less of promised benefits -- to pay an additional premium. This change would up collect an additional $2.5 billion annually.

"Our view is the plans should be fully funded and that means 100 percent funded," Ann Combs, assistant secretary for employee benefits security at the Labor Department, which overseas the PBGC, told CNN/Money.

Combs said the administration proposal would also allow tax-deductible contributions to plans that are overfunded. But with only 20 percent of companies making the maximum allowable contribution, she's not sure how much that will change the outlook.

Watson Wyatt's Coronado said allowing contributions to overfunded plans in the good times would help. "When they limited contributions to overfunded plans, lo and behold, overfunding disappeared," she said.

But she questioned the increased premium and rule changes, saying they would fall too heavily on companies with healthy plans, putting more pressure on big companies to stop offering this kind of benefit.

"It's not sensible to ask profitable plan sponsors to fill in the hole," she said. "If they leave the system, then PBGC has no profitable sponsors to fund the system and we'll end up with a government bailout anyway."

The administration also is proposing that underfunded plans at companies whose debt is not investment grade not be allowed to raise benefits, a proposal that is sure to stir opposition from both union groups and some businesses.

For a look at news, questions and answers about retirement planning, read CNN/Money's Retirement Essentials.