WASHINGTON — Lax reporting rules created by Congress, coupled with corporate America's eagerness to take advantage, have left millions of workers' and retirees' pension plans underfunded without their knowledge, senators were told yesterday.

United Airlines may have set an unsavory example for others in the airline industry, Senate Finance Committee members were told during a hearing on Capitol Hill. After declaring bankruptcy in 2002, the airline won court approval last month to shed $9 billion in pension obligations, shifting responsibility to the federal Pension Benefit Guaranty Corp.

That has contributed to a $23.3 billion deficit at the agency, which insures private pension plans, and triggered fears of another massive taxpayer bailout similar to the 1980s savings and loan crisis. The agency's head told senators the number of pension plans more than $50 million short of promised benefit levels has risen from 221 in 2000 to 1,108 in 2004. Those funds have an average of just 69 percent of promised benefits on hand.

"Unfortunately, when it comes to pension funding, too many high-risk companies do what is legally permissible — rather than what is right — when deciding how much money to put into their pension plans," said Comptroller General David Walker, head of the nonpartisan Government Accountability Office.

The statistics and comments prompted calls for swift legislative action this year, on bills pushed by President Bush, Sen. Jay Rockefeller, D-W.Va., and Rep. John Boehner, R-Ohio. In general, the bills create schedules to eliminate funding shortfalls and revise rules that allow companies to conceal underfunding. They also provide greater transparency so that information on the funds now available only to the federal pension guaranty program is shared with the public.

"The facts are alarming. The time to act is now. Tinkering with the current rules won't do. Another temporary Band-Aid won't do," said Sen. Charles Grassley, the Iowa Republican who chairs the Finance Committee.

About 34 million people — roughly 20 percent of the nation's work force — expect to receive payments from employers through defined benefit plans.

The risk those workers face was highlighted by the United ruling, in which the Pension Benefit Guaranty Corp. assumed responsibility for paying pensions to 120,000 current and former airline employees. Although they were owed more than $9 billion in pension benefits, they will receive only about two-thirds of that amount — $6.6 billion — because of the agency's insurance limits.

An agency report released yesterday showed that pension rules allowed United to underfund its plan without notifying its employees, paying extra insurance premiums or accelerating its pension payments.

United Chairman Glenn Tilton, the focus of most of the day's sharp questions, said the airline was forced to seek pension relief when the government refused to grant it a $1.1 billion loan guarantee. While conceding the ruling has caused employee pain, he added: "From the outset of the bankruptcy process, our mission has been to enable United Airlines to succeed as an enterprise. Without success for the enterprise, the rest is an academic exercise."