The former chief counsel of the House Assassinations Committee warned yesterday that the committee would never find the truth behind the assassinations of President Kennedy or the Rev. Martin Luther King Jr. if it continues on its present course.

Openly critical of the committee's new chairman, Rep. Louis Stoke (D-Ohio), former chief counsel Richard A. Sprague said the investigation has been hampered so much by demands for "sensational" disclosures and public "titillation" that he is now convinced Congress cannot handle the inquiry properly.

In his first public statement since he was forced to resign last month, Sprague said he hoped President Carter might consider appointing a special prosecutor to take over the investigations. In light of his own experience, Sprague said he feels that "the Congress of the United States is not the proper agency, to condut an investigation of any crime, much less crimes of murder."

The former Philadelphia prosecutor stopped short of urging that the Assassinations Committee be disbanded. Shortly before his late-morning press conference here, he visited committee staffers and urged them to prove him wrong to "'show everybody . . . that you can keep political influence out of this thing and get a job done.'"

But Sprague continued, "I don't think they can, because what occurs is that the members of Congress want to staff this with their cronies, with their friends . . . Just about every congressman in existence wrote me with his recommendations as to who to put on the staff. I guess . . . I became 'abrasive' because I turned them down, because I was looking for professionals."

Looking relaxed and tanned after a 10-day vacation in Acapulco, Sprague disclosed yesterday that he quit March 29 - the day before a crucial House vote to continue the investigation - primarily because of Stokes' incipient animosity.

Sprague said he hadn't wanted to resign and felt he still had enough committee members behind him "to avoid being booted out," but then sensed that Stokes was getting increasingly annoyed at him for wanting to hold out.

"I figured, 'Why put up with it again?'" Sprague said, recalling his difficulties with former committee Chairman Henry B. Gonzalez (D-Tex.) who had tried to fire the committee counsel in mid-February but was countermanded by the panel's 11 other members.

The fresh pressures for Sprague's ouster arose March 29 because of an unofficial headcount by committee members which indicated that the investigation would survive the next day's vote on the House floor only if Sprague were replaced.

Summoned to a four-hour meeting in Stokes' office the night of March 29, where he found the chairman with Reps. Floyd J. Fithian (D-Ind.) and Rep. Robert W. Edgar (D-Pa.), Sprague said he had already decided to leave at some future date, but residted the pressures to quit immediately, especially in light of reassurances he had gotten earlier that day from committee members.

Reminding Stokes and the others that they were agreed that criticisms of Sprague's conduct and investigating techniques were injustified, the counsel said he told them "it was a sorry commentary for congressmen in this democracy to be knuckling under to the big lie." Sprague then polled other committee members by telephone and was assured of their support.

But "at that point," Sprague said, "I detected that Mr. Stokes was kind of resentful that he was not getting his way." Anticipating continuing difficulties with the chairman if he stayed, the Philadelphian decided to quit. He made it effective immediately despite a "Don't do that" chorus at the last minute from Fithian, Edgar and Del. Walter E. Fauntroy (D-D.C.), who had joined the group.

Sprague was also critical of the committee's calling of Dutch journalist Willem Oltmans to a meeting two weeks ago "just because of the sensationalism arising from the death of Mr. de Mohrenschildt" (George de Mohrenschildt, a Russian refugee who committed suicide last month after reportedly telling Oltmans of a far-flung conspiracy involving de Mohrenschildt, Lee Harvey Oswald, anti-Castro Cubans, the CIA and Texas oilmen).

The former committee counsel said he still feels some sort of investigation is needed in light of uncorroborated information that has been developed, for instance, in interviews with James Earl Ray. But the committee staff, instead of being left alone to conduct a thorough impartial investigation, has been confronted, Sprague said, with constant push for a public hearing to present something to the media, something to titillate the public, something that has the specter of sensationalism . . .

"That is not the way to investigate," he said, "and it totally impedes the search for the truth."