United Airlines said yesterday that it would not contribute to its employee pension plans while it remains under bankruptcy protection. That move could save it more than a billion dollars in cash over the coming year, but pension experts said it signaled the likelihood that United would terminate some or all of the plans.

A full-blown default by United on all four of its pension plans would send tens of thousands of current and future retirees, and billions of dollars in unfunded obligations, to the government's pension insurance program, dealing the program its biggest blow since the government began insuring pensions in 1974. Such a huge default could also set off a chain reaction, prompting other airlines to reduce their own costly pension plans to stay competitive. That, in turn, would worsen the pension agency's finances.

But the rare and drastic measure would also make United much more attractive to the lenders it needs for the financing to help it emerge from bankruptcy. United filed for bankruptcy in December 2002, hit hard by the decline in travel that began even before the terror attacks of September 2001, when it lost two planes, and the struggle to compete with low-cost rivals.

Terminating a pension plan is a drastic measure that requires approval of the bankruptcy court. In a so-called distress termination, the government takes over the failed pension plan's assets and liabilities and uses the money to pay benefits to the retirees, within certain limits. When there are not enough assets in the plan to cover the obligations, the government's insurance program, the Pension Benefit Guaranty Corporation, makes up the difference. Retirees whose benefits exceed those limits would lose some of them.

The federal agency cannot reject such a termination, although it can go to court to try to get more resources from the defaulting company. The company's workers can also try to block such a termination, either in court or by striking. But success could also be failure, because they could drive the company into liquidation, leaving themselves with neither a pension plan nor jobs.

Federal law requires companies that offer pensions to contribute to their pension funds once a year, or quarterly when a fund is severely weakened, as United's funds are. It is highly unusual for a company to withhold contributions, and virtually unheard-of for a bankrupt company that has skipped contributions to revive the neglected plan later on.

"I can't recall that having ever happened," said Gary M. Ford, a former general counsel of the pension agency, and now a managing principal of the Groom Law Group who specializes in employee benefits law as it relates to bankruptcy.

"If a company were planning to reorganize and keep the pension plans, it would most likely decide to put the cash in," Mr. Ford said. "You haven't really saved anything" by skipping a contribution.

"You've just delayed an obligation," he added. "And at the same time, you're going to take a lot of heat from your employees."

Jean Medina, a spokeswoman for United, declined to comment yesterday on whether the airline intended to abandon its pension plans altogether. "We have a lot of analysis to complete" before making any decisions on the plans' future, she said.

At the Pension Benefit Guaranty Corporation, Randy Clerihue, a spokesman, said that a default on a scale of the one that might be looming at United would be unprecedented.

"If United's plans terminate, it would be the largest loss in the P.B.G.C.'s history," Mr. Clerihue said. The largest pension failure so far, that of Bethlehem Steel in 2002, left the government with $3.6 billion in unfunded pensions. But United's four employee pension funds would have a total deficit of $7.5 billion if they were terminated, according to documents filed in bankruptcy court.

Most of that loss would be borne by the agency, but some would be borne by employees, particularly United's pilots, who have been promised unusually rich pensions that exceed the limits of the program's insurance coverage.

The agency finances its operations by charging premiums to the companies that offer pensions.

The agency was originally established to cover basic blue-collar pensions, and Mr. Clerihue said that in most cases when it takes over a failed pension fund, the retirees do end up receiving full coverage. But in the minority of cases where retirees' benefits exceed the maximum coverage, the losses can be severe, he said. The weaker the pension fund, the worse those losses can be, he said.

"This decision to stop making legally required contributions would only magnify the loss," he said. "It could result in bigger benefit cuts for the participants."

The guaranty agency's financial health peaked in 2000, when it posted a $9.7 billion surplus in its single-employer division, the larger of its two divisions, which insures corporate pension plans. The agency then took a pounding when the stock market fell because most companies have invested more than half of their pension funds in stocks.

Its worst losses were concentrated in failing steel company pension plans. As of last year, the agency was running a deficit of $11.2 billion. In March, it disclosed that it had classified $23.4 billion of airline pension obligations as "reasonably possible" to default.

Should United terminate its pension plans, which cover 134,000 active and retired employees, it will have to negotiate replacements with its labor unions, most likely at much less generous levels. Gary Chaison, professor of industrial affairs at Clark University in Worcester, Mass., said United's statement yesterday was the first move in what he expected would be bitter bargaining.

"They'll be working off a sense of anger and betrayal," Professor Chaison said.

Indeed, union leaders at United reacted to the airline's move with outrage yesterday. Although United said it was still considering its options, some of them interpreted the news as a warning that some pension plans would be terminated.

"Current management should explain to us why the flight attendants should continue to support their restructuring, if this is the best they could do," said Greg Davidowitch, president of the Association of Flight Attendants' local at United.

He called United's decision "demoralizing" and said he now thought United would terminate the flight attendants' plan.

That plan does not offer especially large pensions, and most of the flight attendants are probably within the limits of the government insurance program, at least on the benefits they have earned so far. But if the plan is terminated, all of them would lose their ability to earn any additional pension benefits.

S. R. Canale, president and directing general chairman of District Lodge 141 of the International Association of Machinists, said his union had already been meeting with pension lawyers and officials of the pension agency to protect benefits.

"Financing is not the only ingredient to surviving bankruptcy," Mr. Canales said in a letter to members. "A productive relationship with employees is necessary, but that no longer exists at United Airlines."

"We will not simply allow United to proceed without a fight," he added.

United's action came a week after it skipped contributions of $72.4 million that it owed to three of its four pension plans. Its next quarterly contribution, of $92 million, is due in October. In addition, United is scheduled to make a catch-up payment, of $404 million, on Sept. 15, to complete the contributions that it still owes from 2003.

The quarterly payments were to have continued in 2005 and were scheduled to increase after the expiration of a two-year pension breather that Congress granted the airlines last April.

Word that United was suspending its pension contributions came as the airline told a bankruptcy court in Chicago that it had replenished the financing it needed to operate while in bankruptcy - known as debtor-in-possession financing - which was originally arranged soon after its bankruptcy filing in December 2002. It said the newly arranged financing would be good until June 30, 2005.

Originally, United borrowed $1 billion to keep its operations going while it waited for an answer from a federal loan board on its bid for a guarantee package, which it hoped to use as the basis of its restructuring.

But the Air Transportation Stabilization Board turned down United's application on June 28, saying it thought United could find lenders without government assistance. Soon afterward, the airline, which had already cut costs by $5 billion a year, warned union members they had to expect to make further concessions.

Industry analysts said lenders would be unwilling to invest in United, which owes its pension plans an estimated $4.1 billion over the next five years. United acknowledged that yesterday.

"In the absence of a federal loan guarantee, United's long-term business plan must have cash flow and liquidity levels that the capital markets are willing to finance," the airline said in a statement.

It went on, "Because existing pension plan contributions will remain a huge financial burden after exit, it is incumbent on United to study all possible options and to determine whether United can sustain this burden and still attract exit financing." United said the move would not have any day-to-day effect on union members' pension payments.

Robert W. Mann, an industry consultant based in Port Washington, N.Y., said the airline was bowing to reality. "New lenders and new equity don't like to pay old bills," he said.

Last year, United, a unit of UAL, obtained concessions worth $2.5 billion a year in savings from its unions, a few months after it sought bankruptcy protection. But it made few adjustments to its pensions, at the time, a move that may have given unions a false sense of security, said Professor Chaison of Clark University.

"What we saw was Act 1," he said. "Now this is Act 2. It may turn out to be a tragedy. It's a terrible thing for a union to have to negotiate something like this."

For United, the world's second-largest commercial airline, to default on its pension funds would be a historic blow to the labor movement, said David Gregory, a labor law professor at St. John's University in Queens. Pensions have long been one of the three basic things that unions have sought to provide for their members, he said, along with health care coverage and good wages.

"This goes against what unions were set up to provide," Professor Gregory said. The situation is even more notable at United, which was an employee-owned company before it sought bankruptcy protection. Union leaders, in fact, had the power to hire and fire the chief executive. Now, 19 months later, they face seeing management dissolve their retirement plans.

"This is a blow to the concept of employee ownership," Professor Gregory said.