WASHINGTON (AP) _ Congress quietly passed legislation designed to make it harder for Trans World Airlines owner Carl Icahn to give the federal government responsibility for up to $900 million in pensions to company employees.

The measure would allow the federal Pension Benefit Guaranty Corp., or PBGC, to attach Icahn's other financial holdings - such as rail car manufacturer ACF Industries - to cover unfunded pension liabilities as long as he controls the airline, even if he sells most of his stock in it.

''The effect ... is to force Carl Icahn to do his best to keep TWA operating as a going concern,'' said Sen. John Danforth, R-Mo., a prime architect of the provision that was attached to a catchall spending bill sent to President Bush.

''The only way he can avoid immediate liability for the total amount of unfunded pensions is to keep TWA in business.''

Icahn complained to lawmakers through lobbyists that the provision was ''surreptitiously added'' to the bill with no prior notice, hearings or debate.

''We kept this pretty quiet,'' said one congressional aide, echoing remarks from several others that Danforth and other lawmakers who had worked on it for months did not want to tip their hand to Icahn.

Icahn owns more than 80 percent of TWA, which has a debt of $2.5 billion. When he first learned of the measure, he said it ''threatens the existence of TWA and its 28,000 employees.'' But later, an airline spokesman said only that ''it was unnecessary.''

A ''pre-packaged agreement'' being negotiated with TWA's creditors for a Chapter 11 bankruptcy reorganization envisions Icahn trading half to three- fourths of his stock to creditors in exchange for a $1 billion reduction in the airline's debt.

Unfunded pension obligations would be the bulk of the remaining debt. And under the 1974 Employee Retirement Income Security Act, Icahn would have escaped much of the liability for them with his reduced ownership in TWA.

TWA's unions, business leaders in its hub city of St. Louis and the PBGC all breathed a sigh of relief when Congress on its last day in session this year closed the main channel for doing it.

Culminating a clandestine effort of several months, Danforth and House Democratic Leader Richard Gephardt of St. Louis quietly slipped a change in the pension law into a $6.9 billion emergency aid bill for farmers, disaster victims and the Soviet Union just hours before its passage Nov. 27.

''The bill sends a strong message to Mr. Icahn that he cannot escape his pension liabilities through clever reconstruction of his empire,'' said Diane Burkley, executive director of the PBGC.

Operating much like the government's bank deposit insurance fund, the PBGC guarantees retirees at least a portion of the promised benefits even if their pension plan collapses.

The self-financing agency collects a fee from pension plan sponsors for each participant. It guarantees paying retirees at age 65 up to $27,000 of the annual benefits they were promised in a terminated plan.

As of last August, the PBGC estimated it would be on the hook for at least $440 million of TWA's unfunded pension liabilities.

But agency officials, citing their experience with defaulted pension plans of Eastern and Pan American airlines, said last week that its actual exposure could be twice that, particularly if Icahn continues to dismantle TWA by selling off routes and employees opt for early retirement.

Burkley, who has been in negotiations with Icahn for months over TWA's pension problems, said Congress' action ''closes one route'' for him to escape his liabilities.

''It's not the end of the matter from our perspective,'' she added. ''There are still several ways for him to get out from under his obligations. We're going to continue to take whatever actions are necessary.''