New York investor Carl Icahn yesterday offered to acquire the two-thirds of Trans World Airlines Inc. he does not already own for about $546 million in cash and securities, a proposal that threatens the pending combination of TWA and Texas Air Corp.

Texas Air and TWA announced a friendly merger in June that would create the nation's second-largest airline, a deal that has not yet been approved by stockholders. TWA had solicited a takeover bid from Texas Air to escape a hostile offer earlier this year from Icahn, who owns 35 percent of TWA. Icahn said yesterday he will vote his shares against the pending Texas Air-TWA combination.

Texas Air is offering to acquire TWA for $23 a share, or $19 in cash and $4 in securities. Icahn said his offer is superior because it is worth $24 a share, or $19.50 in cash and $4.50 in comparable securities.

Icahn's takeover bid may be an attempt to force Texas Air to buy his TWA shares at a premium price, since Icahn and Texas Air have been unable to reach an agreement on the sale of his shares. His new takeover bid puts additional pressure on the company to buy him out on his terms.

Icahn's takeover bid received the support of several labor unions representing TWA employes. These unions oppose the sale of the company to Texas Air because the company and its chairman, Frank Lorenzo, have reputations as unusually tough labor negotiators.

"The TWA board has already indicated that an offer of $19 in cash and $4 in preferred stock for each share of TWA common stock is fair to shareholders," Icahn said yesterday. "Since I am offering a higher price, have achieved the support of the pilots' and machinists' unions and am on record before federal and state courts, Congress and the Department of Transportation that I will operate TWA in such a way as to maximize the long-term interest of TWA, its employes and the traveling public, I hope the board will act favorably toward my proposal."

Icahn reserved the right to withdraw his offer if the TWA board of directors does not respond favorably by 5 p.m. on Thursday. He also said that depending on the availability and price of TWA shares, he may increase his 35 percent stake in the company, which already includes about 11.7 million shares.

TWA, which has labor costs that are among the highest in the airline industry, would receive substantial wage concessions in exchange for union stock ownership and profit participation under Icahn's plan, he said. He said that after the Texas Air merger agreement was announced, TWA union representatives encouraged him to acquire control of the company.

Icahn said he has reached agreements with the leaders of the Air Line Pilots Association and the International Association of Machinists and Aerospace Workers. The agreements are subject to ratification by union members. The agreements include limitations on the sale of TWA assets and the maintenance of capital expenditures in the event Icahn acquires control of TWA.

The agreements also grant the unions participation in proceeds in excess of specificed amounts if Icahn sells his TWA stock to a third party, such as Texas Air, before certain dates. This unusual aspect of the agreement indicates that the unions believe Icahn's takeover bid may be a ploy to force Texas Air to buy his TWA shares at a premium. Since the unions are aiding Icahn by agreeing to future wage concessions, they want to participate in that premium if he receives it.