An unpublished study by the staff of the Senate Select Committee on Intelligence has concluded that about two-thirds of classified intelligence leaks come from the executive branch, challenging claims by some administration officials that Congress is to blame for news reports that spill government secrets.

The study, which was based on an analysis of eight prominent newspapers in the first six months of last year, revealed that 98 of 147 disclosures of classified information were attributed to anonymous Reagan administration sources.

The conclusions appear to contradict charges by fired White House aide Lt. Col. Oliver L. North and other current and former administration officials that Congress has been the major source of leaks of sensitive national security information.

Administration officials have challenged the study, however, saying that all parts of the government are to blame for what has become a serious problem, and that leaks cannot be quantified.

Congress was responsible for 13 leaks, or 9 percent, in the period studied, the results show. Seventeen disclosures cited sources in the military, outside the government or in a foreign government. In 19 cases, the words used in the articles to describe the sources were not specific enough to determine whether they were administration, congressional or other sources.

Pentagon spokesman Robert B. Sims said the study appeared to make "a lot of assumptions that one can't have too much confidence in."

"I don't think that any part of the government has a patent on {leaks}," Sims said. "Generally we don't find out who the source is. Our own investigators hardly ever can nail it down."

David Holliday, special assistant to the chairman of the Senate intelligence committee, said he asked staff members early last year to research the sources of leaks at a time when the leadership of the committee was feuding with CIA Director William J. Casey over a series of intelligence disclosures.

In a heated exchange, Casey attacked then-chairman David F. Durenberger (R-Minn.) for conducting intelligence oversight in an "off-the-cuff" manner that had resulted in serious intelligence disclosures. Durenberger and Sen. Patrick J. Leahy (D-Vt.), who was vice chairman at the time, charged that administration leaks were creating a national security crisis.

Durenberger's press secretary, Lois West, said that the survey suggests that Durenberger and Leahy were generally correct in last year's confrontation. "We don't take it to be necessarily the last word, just to be an indication," West said. "This is a nebulous area where it's hard to prove anything."

Congressional and administration officials said a possible explanation for the large number of administration leaks is that the executive branch is much bigger than the legislative branch, with more people who have access to sensitive information. Bernard F. McMahon, former staff director of the Senate intelligence committee, said that 150,000 executive branch personnel have high-level security clearance, while only 96 congressional staff members have similar access to government secrets.

Intelligence committee staff who did the study examined only articles on intelligence matters. Holliday said the researchers defined a "leak" as any disclosure of a government secret that was attributed to government officials and not announced in a formal statement.

Newspapers examined in the intelligence committee study were The Washington Post, The New York Times, The Washington Times, the Miami Herald, The Wall Street Journal, The Christian Science Monitor, the Los Angeles Times and The (Baltimore) Sun.

Until this month, when administration officials renewed charges that Congress was to blame for serious unauthorized disclosures, the existence of the committee study was not widely known. Allegations in the Iran-contra hearings prompted Rep. Anthony C. Beilenson's (D-Calif.) office to distribute copies.

North told congressional investigators earlier this month that he had lied to Congress about covert operations and charged that legislators could not keep secrets.

Newsweek reported last week, however, that North had leaked information about the U.S. interception of a plane carrying the suspected hijackers of the Achille Lauro.

White House spokesman Marlin Fitzwater this month criticized House Armed Services Committee Chairman Les Aspin (D-Wis.) for revealing information about reflagging in the Persian Gulf.

Former national security adviser John M. Poindexter backed off from sharp criticism of Congress, however, telling the Iran-contra committees that claims that legislators are solely responsible for leaks are "pure nonsense."