WASHINGTON (AP) _ An engineer who tried to stop last year's ill-fated space shuttle Challenger launch is accusing Morton Thiokol Inc. of punishing him for telling investigators about defective rocket booster seals.

Roger Boisjoly alleges in a $1 billion lawsuit that Morton Thiokol defamed his professional reputation as punishment for giving truthful testimony to the presidential commission and congressional committees that investigated the Jan. 28, 1986 shuttle explosion which killed seven crew members.

The lawsuit was filed here Wednesday in U.S. District Court on the first anniversary of the Challenger disaster. It accuses the company and NASA of conspiring to thwart the investigations as part of a longstanding effort to withhold information about defective O-ring seals and joints between the solid rocket motor units.

The National Aeronautics and Space Administration is not named in the lawsuit as a defendant. But Robert N. Levin, Boisjoly's attorney, said he planned to file a similar complaint with the agency for administrative adjudication.

Boisjoly, 48, of Willard, Utah, contends the space agency was part of the conspiracy with Morton Thiokol that was ''motivated by their desire to keep secret their long-term mismanagement of the joint and seal problem; their mutual interest in withholding from the Congress and the public the many safety problems with the shuttle.''

''NASA consistently lied to the Congress and the public as to the safety record of the shuttles,'' the lawsuit says.

Neither NASA's David Garrett nor Morton Thiokol spokesman Gilbert Moore would comment on the lawsuit, saying they had not seen the 26-page complaint.

Boisjoly told the presidential commission chaired by former Secretary of State William Rogers that he was prevented from talking to NASA investigators and he believed this was the result of retaliation for his testimony.

The lawsuit alleges that Morton Thiokol publicly threatened his job and portrayed him as ''a disgruntled or malcontented employee whose views should be discounted and whose professional expertise should be doubted.''

''Although Thiokol gave lip service to the idea of cooperation, in fact defendant removed plaintiff from any position where he could interact directly with NASA investigators and Thiokol publicly threatened his job, labeling him in press interviews as a 'tattle tale,''' the lawsuit contends.

Boisjoly alleged that since the mid-1970s, NASA and Morton Thiokol concealed from the public and company stockholders information about deficiencies in the seal joints.

A day before the explosion, Boisjoly was among a group of Morton Thiokol engineers who urged NASA not to launch Challenger in cold weather because the joint seals might break.

''Neither NASA nor Thiokol advised the flight crew, the owners of the payloads aboard the Challenger, the insurance companies with coverage involved in the flight nor the public of the extreme hazards presented by the cold nor the unanimous position of the Thiokol engineers that it was grossly unsafe to fly in those conditions,'' the lawsuit says.

It also contends that Boisjoly became disabled from ''post-traumatic stress disorder and depression caused directly by the disaster'' and ''the intentional wrongdoings of NASA and Thiokol.''

Boisjoly, who had been on medical leave from the company since last July, recently was placed on permanent disability.

Besides defamation, Boisjoly accuses Morton Thiokol of anti-trust violations, obstruction of the shuttle investigations, and racketeering under the federal racketeer influenced corrupt organization (RICO) statute.

The RICO count accuses the Chicago-based company of securities fraud for its failure to notify stockholders of its longstanding knowledge about the dangers of the design flaws.