The House Government Operations Committee is investigating NASA's efforts to purchase new shuttle booster rockets amid complaints from aerospace firms that the agency is improperly seeking to perpetuate its contract with Morton Thiokol Inc.

Committee Chairman Rep. Jack Brooks (D-Tex.) and ranking minority member Frank Horton (R-N.Y.) have charged in a letter to NASA Administrator James C. Fletcher that space agency officials have structured their procurement for new boosters in a manner that "clearly shows a heavy bias and favoritism towards" Thiokol and "may be in violation" of federal law requiring open competition on federal contracts.

In a related development yesterday, the White House delayed announcing an expected approval of a $2.8 billion shuttle orbiter to replace Challenger after chief of staff Donald T. Regan objected that the National Aeronautics and Space Administration has not provided adequate justification for it.

At a morning staff meeting, Regan contended that the agency has not made an "adequate response" to questions he had raised at a National Security Council meeting two weeks ago asking why the administration should spend more money on shuttles rather than wait for a new generation of space vehicles, sources said. There is also continued debate on where the funds to pay for the orbiter should come from, with NASA resisting an Office of Management and Budget proposal for corresponding cuts in the agency's budget, the sources said.

"There's been a continued lobbying effort [by NASA] in the press to sell us the shuttle," said one administration official. "But there's got to be a good reason for it. . . . It's basically a matter of them not coming forward with what was asked for."

The official added that no timetable has been set for announcing the decision, but said, "there's nothing that I've seen that indicates this is on a fast track."

The Government Operations Committee's letter -- and a similar protest signed by more than 240 members of the House -- adds fuel to a controversy that has surrounded NASA's sole-source contract with Thiokol since defects in the booster rockets were identified as the cause of the Jan. 28 Challenger accident.

The Chicago-based company has enjoyed a monopoly on the supply of booster rocket motors for the shuttle since 1973, providing the firm with an estimated $350 million to $450 million in annual revenue.

Last December, responding to congressional complaints that the Thiokol monopoly was resulting in higher costs, NASA proposed moving to second-source bidding on purchases of booster rockets, opening up the booster contract to competition for the first time.

The Challenger disaster has set back NASA's timetable for upcoming booster rocket purchases until probably the early 1990s. But in the meantime, potential competitors, including Hercules Inc., United Technologies Corp. and Aerojet Strategic Propulsion Co., have complained to Congress that NASA has made it virtually impossible for them to bid successfully.

Brooks and Horton charged that NASA has guaranteed that 50 percent of any purchases go to Thiokol and that Thiokol would be the subcontractor for the rocket nozzles of remaining purchases, amounting to about 30 percent of the award.

The contract with Thiokol has attracted scrutiny in recent weeks following testimony by two engineers that they were punished by the company after they objected to launching the Challenger because of safety concerns. Company official denied the charges.

Russ Bardos, NASA's manager of shuttle productivity, said the guarantees for Thiokol were justified to lower the space agency's costs since it would otherwise have to pay extensive capital start-up costs, estimated to be as much as $100 million, for a new supplier. Moreover, the Challenger disaster, by slowing down the agency's flight schedule, has lowered the agency's planned production purchases considerably, making it even more expensive to have two competing suppliers, he said.

"The economics of going to second source were not favorable before the Challenger, and it's even worse now because of a lower production rate," Bardos said.