A former FBI break-in specialist has testified that he carried out illegal "black bag jobs" in two of the bureau's biggest recent investigations in the mistaken belief that the federal courts had approved the break-ins.

The agent, H. Edward Tickel, testified in U.S. District Court in Alexandria Thursday that he personally notified FBI Director William H. Webster about the illicit entries in the fall of 1980 and that the director "got very mad" at him for speaking up.

Webster took the witness stand late yesterday afternoon and denied Tickel's accusation. "I'm confident that no one, including Mr. Tickel, ever talked to me about an illegal entry in either of those cases," Webster declared.

The director said he would have ordered immediate investigations if such allegations had been made to him.

Tickel, 42, who is standing trial on criminal charges for the second time in as many months, has pleaded not guilty to charges that he dealt in stolen diamonds, evaded taxes, obstructed justice and urged friends to commit perjury. A 14-year FBI veteran, he was acquitted by a Washington jury last month on separate charges of trying to burglarize the FBI's credit union at the J. Edgar Hoover Building.

Tickel has testified that one of the surreptitious entries took place during Operation Pendorf in November 1979 at a Chicago hotel suite reserved for Roy Lee Williams, then a vice president of the Teamsters Union.

Williams, who was subsequently elected president of the union, was one of the main targets of the investigation. He was convicted last December of conspiring to bribe Sen. Howard Cannon (D-Nev.) and is awaiting sentencing. There was no evidence at any of the stages of his trial and still-unfinished sentencing hearings of a surreptitious FBI entry into his hotel suite.

Tickel, once the FBI's premier break-in specialist, said the other unauthorized entry was made in January 1980 at the Louisiana hunting lodge of New Orleans Mafia kingpin Carlos Marcello. Marcello was the prime target of Operation Brilab, another major FBI investigation.

In both cases, Tickel said he discovered later that the assurances he had received from fellow FBI agents were false and that no court orders authorizing the entries had been issued.

In the Williams' case, Tickel said: "I didn't find out until a year later" that no warrant had been issued. He said he made the discovery when the Chicago FBI office sent an "urgent" memo to Webster asking for Tickel's assistance in installing hidden television cameras in the Teamster leader's hotel suite.

According to Tickel, one FBI agent in Chicago told him over the telephone " 'We're going back in on Roy Williams.' And I said, 'Why?' I advised them that they already had a key" to the suite.

" . . . And so they said, 'We never had a court order.' " At that, Tickel said, he realized that what he had done the year before--making the key and entering the suite--was illegal, "and I was very concerned."

Uppermost in his mind, Tickel said, was the trial in Washington at the time of two once-high-ranking FBI officials, W. Mark Felt and Edward S. Miller, for authorizing illegal break-ins in the early 1970s. Felt and Miller were convicted of conspiracy on Nov. 6, 1980, the day before the message from the FBI in Chicago requesting Tickel's expertise.

Tickel said he confronted Webster in the director's 7th floor dining room shortly therafter, probably in November or December of 1980, and told him about both the Marcello and Williams incidents. Tickel said he was afraid he could be indicted and he figured Webster "could be indicted, too."

"I was extremely concerned, not about him, but myself," Tickel said. The agent said he had always assumed that the entries he conducted for the installation of FBI bugs and wiretaps had been properly authorized.

"I went up there to him in very good faith," Tickel testified. "He didn't take it as good faith. In fact, he got very mad and I left."

Although he was under criminal investigation by the FBI, Tickel said he was still conducting court-authorized entries around the country in November 1980. In fact, he said: "I was working the most important national security case that the Bureau has." Soon thereafter, he said, he was pulled off such work.

Webster "absolutely" rejected Tickel's account. He said he was once photographed with Tickel and other FBI agents in "a group of winning baseball players" in the bureau but didn't know him personally.

Prosecutor John P. Hume pointed out that FBI records indicated that Tickel had been sent to Chicago in 1979 and New Orleans in 1980 only to carry out "technical surveys" before the alleged entries.

Tickel said he had been assured by agents that "they have all the papers" and he said he entered both Marcello's hunting lodge and Williams' suite at the Sheraton O'Hare Hotel without suspecting otherwise.

"Did you do a technical survey or an entry?" defense attorney Kenneth Robinson asked Tickel about the Chicago incident.

"I did an entry," Tickel replied.

Q. If there was not a warrant, would that have been a violation of law?

A. It sure would.

Q. The presentation to the director of the FBI, that you had done a technical survey, is that true or false?

A. False.

Q. Should Chicago FBI have known that?

A. They should have.

The former FBI agent has repeatedly denied all the charges against him. He contends that he is the victim of a bureau "vendetta" stemming from his complaints.

Webster said the first he knew of Tickel's allegations was when he made them in a much more general way at his trial in Washington last month. The director said he subsequently reported them to the Justice Department, but did not know whether the information, in turn, had been conveyed to the judges in the Williams and Marcello cases.