Private investigator Robert A. Maheu was paid by a competitor of Aristotle Onassis in 1954 to carry out a campaign of wiretaps and dirty tricks against the Greek shipping tycoon - with the knowledge and approval of the CIA and then-vice president Richard M. Nixon, according to a Playboy magazine article.

Maheu, a former FBI agent who later became a top aide to billionaire Howard Hughes, was hired by Stavros Niarchos to undermine a lucrative contract Onassis had signed with the Saudi Arabian government to transport oil, writer Jim Hougan says in the September issue of the magazine.

Maheu confirmed the basic elements of the article yesterday in a phone interview from Las Vegas. But he denied Hougan's allegation that his actions were part of an international conspiracy. "That's a crock," Maheu said.

"I wouldn't take the assignment [from Niarchos] until I cleared it with the outfit," Maheu said. The "outfit," he added, was the Central Intelligence Agency. He was on a $500-a-month retainer from the agency at the time, he said.

He reported his anti-Onassis activities regularly to the CIA, Maheu said, including the use of the illegal and "unproductive" wiretap on Onassis' New York office. The CIA also helped his operatives in Rome, where derogatory stories about Onassis were peddled to a newspaper. Onassis eventually lost the contract. He died in 1975.

Maheu also acknowledged briefing Nixon about the campaign to subvert the Onassis contract, but said it came after his extensive "research" for Niarchos. Hougan's report alleges that Nixon gave Maheu and an associate, John Gerrity, the original "Mission: Impossible" assignment.

The Playboy article quotes Gerrity as saying then-Assistant Attorney General Warren E. Burger - now chief justice of the United States - was also kept informed of the anti-Onassis campaign by U.S. intelligence agencies.

As head of the Justice Department's civil division at the time, Burger approved a massive suit against Onassis that same year, which alleged he illegally bought some surplus American ships. Onassis also faced a criminal indictment in the case, but it was dropped later as part of a settlement.

Burger said yesterday through a spokesman that he received no such intelligence reports. The spokesman added that the Justice Department also charged Niarchos, the would-be-benefactor in the plot, in 1954 for similar activities.

Nixon could not be reached for comment on the Playboy article. The CIA refused to comment.

Maheu's involvement in the campaign against Onassis was alluded to, without naming the two shippers, in a footnote in a November 1975 Senate Intelligence Committee report on CIA assassination plots.

That report detailed Maheu's role as a middleman between the CIA and the Mafia in a plan to kill Cuban leader Fidel Castro.

The CIA used Maheu in several sensitive covert actions where it "didn't want to have an agency person or a government person get caught," the report said.

Maheu denied Hougan's contention that Niarchos was fronting for the major oil companies who feared their own monopoly in Saudi Arabia would be threatened by Onassis' deal.

A State Department official arranged the briefing for Nixon, Maheu said, to bring him up to date on the seriousness of the Onassis contract. A few days later in June, he added, the State Department issued its first public protest about the arrangement.

In the meantime, Gerrity went to Europe to spread derogatory stories about Onassis by bribing reporters, the Playboy article said. Gerrity, a former Washington Post reporter and now a local financial correspondent, could not be reached for comment.

In Rome, according to the Hougan story, Gerrity had two CIA agents at his command. "I wasn't a CIA agent - the CIA was my agent," he was quoted as saying.

The Hougan article is part of a forthcoming book about the use of secret agents by private corporations and government.

"We were always being reminded that the CIA was behind the operation, that it was government work," a Maheu associate was quoted as saying.

While confirming that he kept the CIA constantly informed, Maheu insisted that he was paid only by Niarchos for the anti-Onassis campaign. He said an estimate that his bill was $187,000 was "peanuts."

"I had four or five men in the Mid-east on that one time," he said. "I don't recall what the total was but it was a lot more than that."