Sweeping reforms in security at Britain's top-secret electronic intelligence organization have been introduced to prevent any recurrence of the long-term penetration of the agency by a Soviet spy, Prime Minister Margaret Thatcher announced today.

Thatcher released the findings of an official investigation ordered last fall after the espionage conviction of Geoffrey Arthur Prime, a Russian-language specialist for Government Communications Headquarters who pleaded guilty to passing secrets to the Soviets over a period of 15 years.

The probe included extensive interviews with American intelligence officials.

The commission, under the chairmanship of Lord Bridge of Harwich, confirmed that Prime's betrayal of British electronic intelligence techniques to the Soviets had inflicted "damage. . .of a very high order . . .greatly magnified by the fact that United States secrets had been no less gravely compromised than our own.

"Nevertheless," the report said, the affair had "not irrevocably dashed the extremely close relationship between Britain and the United States in the intelligence field."

"In particular," the report said, Prime passed the Soviets "a wealth of information" concerning the operations of the U.S. National Security Agency, the nation's main recipient of electronic and signals intelligence.

Among the measures adopted immediately at Government Communications Headquarters and Britain's other intelligence and security agencies are closer scrutiny of prospective employes, including the use of psychological tests and extensive interviews, random searches and tighter control of secret documents.

Throughout his employment at the agency from 1968 to 1977, the report disclosed, Prime was able, without hindrance, to take secret material to his home and send its substance to the Soviets. Over the years, Prime was also subjected to four security clearances that failed to turn up any suspicious signs, the report said.

Asserting that the only means of protecting the agency with "any confidence" would be the use of a lie detector test of prospective and current employes, Thatcher said a pilot study of polygraphs is being undertaken. She said that the "invasion of. . .privacy" involved in such measures is necessary in dealing with people "who have access to the nation's most sensitive secrets."

But Thatcher also accepted a commission recommendation that the results of a polygraph alone should not be regarded as sufficient grounds to withhold a security clearance.

Prime was recruited by the Soviets while serving with the Royal Air Force in West Berlin in 1968 and had his last known contact with Russian agents in 1981. The report said that the commission had been "unable to eliminate the possibility" that Prime was not the only Soviet agent working at the agency during that period, but said its investigation had "produced no positive evidence to that effect." That aspect of the probe is continuing.

The report was prepared by the Security Commission, a senior panel that monitors the management of Britain's intelligence organizations. The scope of the report's recommendations indicates the existing system was found to be far too lax.