One of Indian Prime Minister Rajiv Gandhi's most senior aides, principal secretary P.C. Alexander, has resigned following the breakup of an espionage ring that is said to have penetrated top offices of the prime minister's office, the Defense Ministry, the Home Affairs Ministry and the president's office, government officials said today.

Alexander, who had been the closest adviser of assassinated prime minister Indira Gandhi, resigned after his personal secretary, T.N. Kher, was arrested and charged under the Official Secrets Act for passing classified defense information to agents of an unspecified foreign country.

The government imposed a veil of secrecy around the case, the latest of a series of secrets-for-sale scandals involving public officials in recent years, and spokesmen refused to say to which country nine officials arrested had passed information.

Gandhi's press adviser, Shadara Prasad, while confirming Alexander's resignation, would not discuss details of the case. But officials made no suggestion that Alexander was actively involved in the secrets leaks and hinted that he may have resigned over the embarrassment caused by the security lapse.

News agencies reported that 11 persons -- two businessmen with links to multinational corporations and the nine government officials -- had been arrested by the counterespionage branch of the Intelligence Bureau. One of the officials was identified as S. Shankaran, senior personal assistant in the press office of Indian President Zail Singh.

The Press Trust of India said that the Research and Intelligence Wing, a secret service of the prime minister's secretariat, had first uncovered the espionage network and that the arrest of Kher, which touched off a chain of other arrests yesterday, had followed 10 months of surveillance.

Official sources in the Indian government said that "extremely sensitive and classified" information on defense was passed on to foreign agents. The Press Trust of India said that trunkloads of photocopied documents had been seized at the homes of some of the suspects.

The documents were said to cover purchases of sensitive defense equipment and delivery schedules, as well as the daily intelligence reports sent to the prime minister by the cabinet secretariat.

The only attempt to implicate a specific foreign power came today from the Communist Party of India (Marxist), whose general secretary, E.M.S. Namboodripad, told reporters the spy ring was part of an "American conspiracy." He offered no details to support his allegation.

A U.S. Embassy spokesman said, "We never comment on intelligence matters," and refused to discuss details of the case.