JERUSALEM, JULY 8 -- Amiram Nir, a key Israeli figure in the Iran arms deal, has been stripped of authority in his post as terrorism adviser to Prime Minister Yitzhak Shamir because he allegedly lied to Shamir about a secret meeting that he held earlier this year with Manucher Ghorbanifar, another key player in the affair, according to informed Israeli sources.

Shamir has not fired Nir because such an action might be interpreted publicly as an admission that Israel erred in its involvement in the arms affair, which officials here have steadfastly denied. But the sources said Shamir no longer has confidence in Nir's credibility and fears that he may have lied about other matters in the Iran-contra affair as well.

Nir's credibility is of crucial importance to Israel because officials here have relied almost solely on his account in their denial of allegations by former National Security Council aide Lt. Col. Oliver L. North that Israel either originated, knew about or approved the plan to funnel profits from the arms sales to the rebels, or contras, fighting the Nicaraguan government.

"It's Nir's word against North's, and the state of Israel has chosen to believe Nir," said one source.

North told a joint congressional investigating committee in Washington today that he believed Ghorbanifar, an Iranian middleman in the arms deal, was an Israeli agent and that the Iranian had proposed the diversion of funds from the sales to the contras in a meeting in January 1986.

North testified that he believed the proposal was made "with the full knowledge and acquiesence and support, if not the original idea, of the Israeli intelligence service, if not the Israeli government."

Israeli officials have strongly denied that they knew anything about the contra diversion, and they refused to comment tonight on North's testimony.

"We have nothing to add to what we've said in the past, and it's not in our interest to come in after every testimony and deny it again," said a senior official.

Nir has refused to comment publicly and could not be reached for comment tonight.

But Nir has told an internal review panel here that he did not originate the contra funds diversion and had no knowledge of it.

Israeli officials have said they do not believe that North, an avid supporter of aid to the contras, needed or sought Israeli prompting for the diversion scheme, although many suspect that North told Nir about the plan.

The two men, who served in parallel posts in their governments, reportedly developed a close kinship, seeing themselves as prime warriors in their countries' campaigns against terrorism.

Nir was appointed to the terrorism adviser's post by prime minister Shimon Peres in 1984. He took over Israel's end of the Iran arms connection in December 1985 after North and other American officials expressed dissatisfaction with the Israelis who first ran the operation: businessmen Al Schwimmer and Yaacov Nimrodi and former Foreign Ministry official David Kimche.

Those three have said that Nir helped force them out of the operation, and they have been privately critical of his role in the affair, saying things got "out of hand" after he took over.

Kimche, a former senior official of the Mossad spy agency, has criticized combining the arms-for-hostages operation with the scheme to aid the contras, saying it violated a fundamental rule of intelligence that says covert operations should be kept separate to reduce the risks of detection.

When Shamir took over the premiership in October 1986, he kept on Nir at Peres' urging because he was told that Nir was a crucial player in the arms channel, informed sources said.

They said Shamir had misgivings because Nir was closely linked to Peres, a political rival, and because senior officials of Israel's intelligence services did not trust Nir, who was a television journalist before joining Peres' government.

Shamir's misgivings were confirmed last March when the prime minister learned that Nir had met that month with Ghorbanifar in Geneva, informed sources said.

When confronted on the meeting, Nir at first said he had met Ghorbanifar by chance, but then later claimed to have obtained permission from Azriel Nevo, Shamir's military secretary, the sources said. But Nevo said he had only granted Nir permission for a family vacation in London, they said.

Shamir, himself a former intelligence agent, is said to believe that Nir met Ghorbanifar to coordinate their "cover stories" about the arms operation. The prime minister, angered by Nir's deception, then decided to strip Nir of his duties while keeping him in his post.

"Amiram Nir has the safest job in Israel -- nobody can afford to fire him -- but all he is doing is warming his chair," said one source.

Israel traditionally refuses comment on the identity of its intelligence operatives, and no official tonight would respond to North's allegation that Ghorbanifar was an Israeli agent.

But several sources noted that the Mossad generally had been kept out of the Iran arms connection, which was run first by Israeli businessmen and later by Nir in a pattern that echoed the White House modus operandi of "privatizing" the operation. These sources said they doubted that Ghorbanifar was a Mossad agent.

"Maybe he was working for us, maybe for the Iranians, maybe for both," said one source. "Undoubtedly he was also working for himself."

Nonetheless, Israeli officials and intermediaries -- including Nir, Kimche, Nimrodi and Schwimmer -- repeatedly vouched for Ghorbanifar's credibility and reliability despite the fact that he failed three lie detector tests in the United States and was branded as untrustworthy by the Central Intelligence Agency.

"You don't make these kind of deals with the rabbi's son," Kimche has said. "You deal with the people who have the contacts, and Ghorbanifar had the contacts."