White House lawyers told U.S. District Court Chief Judge John J. Sirica yesterday that an 18 minute segment of another of President Nixon’s subpoenaed Watergate tapes is blank.

White House special counsel J. Fred Buzhardt said that “the phenomenon” cropped up in the midst of a recorded conversation between Mr. Nixon and former White House chief of staff H. R. (Bob) Haldeman.

The meeting took place in Mr. Nixon’s Executive Office Building suite on June 20, 1972, just three days after discovery of the break-in and bugging at Democratic National Committee headquarters here. According to former Watergate Special Prosecutor Archibald Cox’s subpoena, “there is every reason to infer that the meeting included discussion of the Watergate incident.”

Dismayed at the report of another missing coversation, Judge Sirica gave the President until Monday to submit all the extant recordings that Cox subpoenaed for safekeeping at the U.S. courthouse here.

If Mr. Nixon is unwilling to do that voluntarily, Sirica said, he would ask Watergate prosecutors to issue a fresh subpoena for the full reels of tape containing the disputed conversations.

Judge Sirica said he was taking the step “not because the court doesn’t trust the White House or the President,” but “in the interest of seeing that nothing else happens” to the still-secret tapes.

Sirica had been planning to leave the original recordings in White House custody until a panel of experts completed a series of tests for any signs of tampering, a process that is expected to take several weeks.

Buzhardt said the June 20 discussion breaks off into what he described as “an audible tone and no conversation” for an 18-minute interval. He said there was conversation between the President and Haldeman both at the beginning and at the end of the unrecognizable portion.

Watergate prosecutors who briefly monitored the recording before yesterday’s hearing said it is “partially obliterated.”

Buzhardt did not contest that description. The White House lawyer said he was told that whatever was said between the President and Haldeman in those 18 minutes has been lost forever. “It is my understanding that it cannot be gotten back,” Buzhardt told reporters after the hearing.

The startling new disclosure came just one day after Mr. Nixon assured the Republican Governors Association in Memphis that the GOP would not be hit with any more bombshells in the Watergate case.

“If there are any more bombs, I’m not aware of them,” the President told the governors Tuesday at their winter conference.

Buzhardt acknowledged that the President knew of the problem with the June 20 tape when he made that remark. The White House lawyer said the missing segment was discovered last Wednesday, Nov. 14, when he and other attorneys for the President were playing back a copy of the original tape to compile an analysis and index that had been ordered by the courts.

The President, Buzhardt said was told “shortly thereafter.”

Buzhardt’s remarks indicated that the White House at least briefly contemplated taking the legal position that the June 20 meeting with Haldeman was not covered by the Cox subpoena because of what Buzhardt called its “ambiguity.”

Cox’s subpoena, Buzhardt said, described the June 20 session as just “one meeting” between the President, Ehrlichman and Haldeman. Actually, the White House lawyer said, there were two sessions, the first between the President and Ehrlichman and the second between the President and Haldeman.

The sessions had been prefaced by a get-together earlier that same day at the White House between Haldeman, Ehrlichman, former Attorney General John N. Mitchell, former White House Counsel John W. Dean III and reportedly Attorney General Richard G. Kleindienst.

“This was their first opportunity for full discussion of how to handle the Watergate incident,” Cox said in his subpoena, “and Ehrlichman has testified that Watergate was indeed the primary subject of the meeting.

“From there,” the subpoena said, “Ehrlichman and then Haldeman went to see the President. The inference that they reported on Watergate and may well have received instructions, is almost irresistible.”

White House aides had also been busy the day before these sessions. On June 19, Dean has said, he spoke to several people, including Watergate conspirator G. Gordon Liddy.

According to reported logs of Mr. Nixon’s meetings on June 20, the President met with Ehrlichman from 10:30 to 11:30 a.m. The meeting with Haldeman, including the 18 blank minutes, apparently lasted from 11:30 a.m. to 1 p.m.

The tape of those conversations was apparently the first one that Mr. Nixon’s personal secretary, Rose Mary Woods, attempted to transcribe in September at Camp David. She said it eventually took her more than 30 hours to get through it.

Miss Woods testified earlier this month that she could not recall the date of the tape that caused her so much trouble, but she said, “It was the first one on the list . . . It was between the President and Ehrlichman chiefly and Haldeman briefly. It was on all sorts of things. It must have been a two-hour or three-hour meeting.”

The only meeting fitting that approximate description on Cox’s subpoena list took place on June 20, 1972.

In her testimony, Miss Woods said the tape was of “very bad” quality. But she did not mention any long blank spot.

Buzhardt said the White House conducted “a large number of technical tests” with the problem tape Tuesday, but all were “unsuccessful.”

“We don’t know what the character of the problem is,” Buzhardt told reporters. The cause, he said, is also unknown.

The White House had already told Judge Sirica last month that two other subpoenaed conversations -- a phone call from the President to former Attorney General Mitchell on the evening of June 20 and an April 15, 1973 meeting with Dean -- had never been recorded.

The Washington Post subsequently reported being told by five different sources that difficulties had arisen concerning the quality of the seven other tapes Cox had subpoenaed.

Except for the June 2- conversation with Haldeman, however, Buzhardt told the court yesterday that “all of the other tapes subpoenaed have been audible throughout.”

Judge Sirica was first told of the missing segment in a chambers conference yesterday afternoon. He then ordered a report made in open court.

Sirica said he was surprised by the disclosure and called it “just another instance” that convinced him he should take some steps to preserve the rest of the recordings. He proposed putting the six-hour reels of tape containing the disputed conversations in a courthouse safe under round-the-clock guard by two deputy U.S. marshals.

Assistant Watergate Special Prosecutor Richard Ben-Veniste said the report that the June 20 conversations with Haldeman is “partially obliterated” would get intensive scrutiny.

White House officials briefly played back the original tape of the Haldeman meeting for Watergate prosecutors yesterday morning to pinpoint the conversation-less segment.

At the hearing before Judge Sirica, Buzhardt protested production of the full six-hour reels on grounds that they include many other conversations not covered by the subpoena, but Sirica did not seem persuaded. He said he did not intend to listen to them.

Judge Sirica said he would resume his hearings on the tapes at 10 a.m. Monday. Meanwhile, he announced the appointment of a six-member panel of experts “to study the authenticity and integrity” of the tapes.

Selected jointly by the White House and the Watergate prosecution force, the panel includes:

-Richard H. Bolt, chairman of Bolt, Beranek and Newman, Inc., a Cambridge, Mass., firm that specializes in acoustics and computer technology.

-Franklin Cooper, an adjunct professor of linguistics at the University of Connecticut, a fellow of the Acoustical Society of America, and a former research engineer for General Electric Research Laboratories.

-James L. Flanagan, an electrical engineer now with the Massachusetts Institute of Technology and a former Bell Laboratories specialist in digital coding of speech and acoustic measurements.

-John G. (Jay) McKnight, an electrical engineer and audio systems consultant.

-Thomas G. Stockham Jr., an associate professor in the University of Utah’s computer science department.

-Mark R. Weiss, vice president for acoustics research of the Federal Scientific Corp. of New York City.

Sirica said their testing would involve physical and electrical measurements of the tapes and should be completed in January.