A Los Angeles attorney said yesterday that he has custody of three videotapes showing the late presidential confidant Alfred Bloomingdale, his longtime mistress Vicki Morgan, two Reagan administration appointees and a congressman participating in several different "sex parties," but authorities said they are unable to confirm existence of such tapes.

Robert K. Steinberg said in a telephone interview that he has tapes that allegedly show "prominent appointed and elected officials and prominent businessmen . . . people who the president knows . . . . Some of them are household names."

Steinberg said he briefly represented Marvin Pancoast, 33, who has been charged with beating Morgan to death with a baseball bat last Thursday while she slept in the condominium they shared.

He said one of Morgan's woman friends, whom he would not identify, brought him the videotapes for possible use in Pancoast's defense.

Steinberg said he plans to call the White House today, and added, "I'm destroying the tapes unless I'm ordered not to."

He said he is withdrawing from Pancoast's defense in part because he believes the tapes have no bearing on the case.

Arthur Barens, a new attorney hired by Pancoast's mother, said in a telephone interview that he is "skeptical" of the tapes' existence and doubts that such tapes would be of any use in Pancoast's defense.

"When you hear the names of the people allegedly on the tapes, I don't think they're the types of people who place themselves in compromising positions," he said.

Steinberg said the alleged tapes include pictures of six men--two currently serving as appointees of President Reagan in positions of less than Cabinet rank, one member of Congress, Bloomingdale and two businessmen.

He said Morgan appeared twice on one of the tapes and that he did not know the two other women in the tapes.

Steinberg said he would show the tapes to Barens and turn them over to court officials if Barens thinks they are relevant to Pancoast's defense.

Steinberg said the only reason the tapes might be relevant is because they might help explain the mental state of Pancoast, a former mental patient. Pancoast said in interviews last weekend that he killed Morgan because she treated him like a "little slave."

Barens said Pancoast plans to plead not guilty by reason of insanity. His scheduled arraignment was postponed yesterday until July 25 to give Barens time to have his client's mental competence evaluated.

Lt. Dan Cooke, a spokesman for the Los Angeles Police Department, said the chief detective working on the Morgan murder case told him that no police officials were aware of the existence of such tapes until reporters began making inquiries yesterday.

Steinberg said two other persons, whom he would not identify, viewed the tapes with him. One, he said, is a Justice Department official who works on the crime task force attached to the U.S. attorney's office in Los Angeles. Steinberg said the Justice official told him that he had no plans to inform his superiors about the tapes because no crime was committed in making such tapes.

Officials in the U.S. attorney's office in Los Angeles said they are unaware of the tapes' existence.

Steinberg said the three tapes are about 40 minutes to an hour in length and that, based on the apparent ages of several participants, the tapes appear to be less than five years old.

Attorney Marvin Mitchelson, who represented Morgan in her $11 million "palimony" suit against Bloomingdale last year, said he did not recall Morgan's telling him of the existence of such tapes.

Mitchelson said Morgan told him she knew about the sexual activities of some Reagan administration officials because of her close relationship with Bloomingdale, a close friend of Reagan and former member of Reagan's "Kitchen Cabinet." He died of cancer last August at age 66.

Last night, Barens noted contradictions between Pancoast telling him that he no longer wanted Steinberg as his attorney and a claim by Steinberg that Pancoast called him from jail to keep him on the case.

"I cannot understand why Steinberg would have brought up these tapes if his intent was to destroy them and if he had any legitimate purpose," Barens said.

Steinberg said that he is not seeking publicity and that reporters had called him about the tapes. He said he does not feel that "private sexual lives" of prominent officials and businessmen "should be splashed all over the newspapers, if it doesn't affect their performance . . . especially when it can't help the defense." Steinberg said the tapes depict sadomasochistic sex, and in at least one instance included multiple partners.

In Morgan's lawsuit against Bloomingdale, continued against his estate after his death, she claimed that for 12 years she had served as a therapist to help Bloomingdale, the founder of the Diners Club and heir to the Bloomingdale's department store chain, "overcome his Marquis de Sade complex."

Morgan testified that she often watched as other women stripped, let Bloomingdale bind them with neckties and then crawled on the floor as he rode on their backs and beat them, while he drooled.

Morgan's suit was filed after Betsy Bloomingdale, a close friend of Nancy Reagan, discovered her husband's affair with Morgan and then stopped his $18,000 monthly payments to Morgan while he was hospitalized.

Morgan, a former model who was 30 when she died, met Pancoast in 1979 and had been living with him for three weeks. Police said the couple had planned to move into separate apartments.