New York ? On the morning after United Airlines won the right to walk away from his pension, Jerry Jedynak woke up groggy and depressed.

“Imagine being with a woman for 31 years and having that relationship shattered one day to find out you’ve been lied to and cheated,” said Jedynak, a customer service agent at Chicago’s O’Hare International Airport since 1974. “You’re going to ask yourself, were you a fool? Why didn’t I see it coming?”

Jedynak and his coworkers at United aren’t the first to feel betrayed. First in steel, now in airlines, tens of thousands of workers and retirees counting on promised pension checks have watched troubled companies turn over the responsibility for underfunded benefit plans to the federal government.

The dilemma now confronting United workers, after a ruling late Tuesday by a federal bankruptcy judge, eventually could be repeated at other airlines, and there are growing worries about underfunded retirement plans maintained by auto and auto parts makers.

But for most other workers, the more likely threat is that their healthy companies with relatively sound pension plans will end the plans and freeze benefits — often as part of a switch to a 401(k) plan — a route already taken by IBM Corp., Avaya Inc. and hundreds of other employers.

That change, long in the making and increasingly widespread, is chipping away at long-standing expectations about retirement, even as lawmakers debate what to do about the other underpinning of life after work — Social Security.

“(Tuesday’s) ruling is a real landmark, not only for these industries, but for American culture,” said William Rochelle, a New York corporate bankruptcy attorney. “What we have in politics and business today is an assault on the well being of retirees.”

The demise of pension plans has accelerated during the past decade, as companies increasingly shifted away from the expense and unpredictability of traditional retirement plans and moved to 401(k)s and other defined contribution plans.

The federal government’s Pension Benefit Guaranty Corp. has seen the number of traditional pension plans it insures drop from a peak of about 114,000 in 1985 to 31,000 last year. The number of people covered by those plans remains relatively stable, but the balance has shifted dramatically.

In 1985, there were 3 1/2 active workers for every retiree covered by a plan. Today, the numbers are equal, reflecting the fact that employers have eliminated plans for many of their new workers, and are coping with obligations owed to longer-living retirees.

Situations like the one at United, a unit of UAL Corp., reflect a somewhat different dynamic. It began in the steel industry in the 1990s, as companies staggering under global competition sought to restructure and free themselves of the weight of retirement obligations, often mandated in union contracts. One after another, companies like LTV, Bethlehem Steel and others jettisoned their pension plans under the protection of a bankruptcy court, turning them over to the PBGC.

The government agency makes good on many smaller pension checks. But it sets a cap on benefits, limiting payout to formerly high-paid workers who were expecting larger pension checks and freezing the accrual of new benefits to workers still years away from retirement.

That progression has continued in the airline industry, with a bankruptcy judge’s permission for US Airways Group Inc. to turn its pension plans over to the government. With United now also dumping its obligations, the pressure grows on other established carriers to follow suit. Indeed, Delta Air Lines Inc. warned this week that it, too, may seek bankruptcy protection.

Pension industry experts are reluctant to predict whether other companies or industries might follow suit. But most acknowledge they are watching General Motors Corp. and Ford Motor Co. — both of whose bond rating were downgraded to junk status last week by Standard & Poor’s.

A recent report by Lehman Brother estimated that GM’s projected benefit obligations are nearly five times the company’s entire market value, a ratio second only to Delta. Ford, auto parts maker Visteon Corp. and Goodyear Tire and Rubber Co. are also in the top 10.

“We’re getting a little nervous about the auto parts and auto industries,” said Ron Gebhardtsbauer, senior pension fellow at the American Academy of Actuaries.

The uncertainty is part of a larger concern at the PBGC, which tracks pension plans it might be forced to take over in the future. It focuses on companies with underfunded pension plans whose bonds are rated at junk status or below.

Just four years ago, the agency estimated that those employers collectively were running pension shortfalls of about $11 billion. But the tally has now soared to $96 billion, a figure considered by the PBGC as future possible losses.

“Our studies have shown that there’s a very high correlation between a junk bond status and the termination of an underfunded pension plan,” said Jeffrey Speicher, a spokesman for the PBGC.

But James F. Hendricks, a senior partner at the law firm Fisher & Phillips LLP in Chicago, argues that companies outside of the airline industry will have a much harder time terminating their pension obligations.

Airline workers are hemmed in, he said, by the Railway Labor Act, which says they cannot go on strike without first going through mediation, and by the fact that even threatening a strike could worsen their situation if nervous travelers started booking flights on other airlines.

Workers in other industries have more leverage, Hendricks said. “They could say ‘Well, if you want to mess with our pension plan, we’re going to strike.’ “

That doesn’t mean other companies won’t exit their pension plans. But that will likely happen at strong companies, observers say. Employers are free to end a pension plan at any time, as long as they make good on benefits already earned.

Many companies have already made that move, citing the double whammy of plummeting stocks and low interest rates in the early part of this decade as making it too difficult for them to predict and pay for their future pension obligations.

“It is the unpredictability of the future, more than the actual costs, that is driving more companies out of the defined benefit system and it is the fact that we might lose these well-funded plans that ought to be raising people’s concerns,” said James Klein, president of the American Benefits Council, which represents firms that operate the largest pension plans.