The Reagan administration, in its first major decision regarding atomic weapons proliferation abroad, is ending the 18-year-old U.S. nuclear co-operation agreement with India.

Informed official sources said the substance of the decision was made known to the Indian government last week by senior State Department officials in high-level talks here that centered on nuclear issues

The Reagan administration is reported to have come to this decision reluctantly, arguing to the Indians that it was "painted into a corner" by nuclear supply policies of the Carter administration and by the 1978 Nuclear Non-proliferation Act, which provides little leeway for continuation of atomic fuel supplies to India.

The conditions for terminating the longstanding and highly controversial nuclear relationship between the two countries were a matter of sharp dispute in the talks last week, the sources said, and will be addressed in another round of U.S.-Indian nuclear talks in New Delhi planned for this summer.

Because of the sensitive nature of the questions still to be decided, including the disputed issue of continued safeguards over U.S. nuclear fuel previously supplied to India, the U.S. decision was not announced by either side in last week's talks.

Reports' questions in both capitals were turned aside by statements that "discussions are continuing." In fact, according to well-informed sources, the basic U.S. decision has been made and communicated to the Indian side.

One report said that an unsigned U.S. diplomatic communication known as a "nonpaper," a device used to assure maximum confidentiality, was handed to Homi N. Sethna, India's top nuclear official, and Eric Gonsalves, the country's foreign ministry secretary, in last week's talks.

Indian sources conceded that New Delhi has been increasingly unhappy with the workings of the U.S. nuclear supply relationship but said that India did not seek its termination. New Dehi as well as Washington is seeking to contain the damage to overall relations from the end of nuclear cooperation.

The U.S. relationship with India predates the formal 1963 pact that promised U.S.-enriched uranium for the U.S.-manufactured nuclear powr reactor at Tarapur. For example, U.S. "heavy water" sold to India in 1956 may have played a role in the May, 1974, Indian atomic blast that sent shock waves through to developed and developing world alike.

It was the 1974 Indian explosion, and India's continuing refusal to rule out further blasts, which has made the U.S. nuclear relationship difficult and complex.

Current U.S. policy is complicated by recent intelligence reports of renewed activity at Pokharan, the test site in the Rajasthan desert where the 1974 underground explosion was staged. Indian officials have disclaimed any plans for a new blast in the immediate future.

It is believed likely that New Delhi will respond, perhaps dramatically, if its neighbor and recurrent foe, Pakistan, is successful in its continuing nuclear weapons drive. Some Indian officials have expressed belief that Pakistan may have a nuclear explosion capability as early as this summer, though other officials are highly skeptical.

Under heavy pressure from India, the Carter administration convinced the Senate last September by only a two-vote margin with extensive lobbying to permit the shipment of 38 tons of enriched uranium for the Tarapur reactor. t

The Reagan administration's decision to end fuel shipments to India is reported to arise in part from grave doubt that Congress would approve any additional flow of uranium.

The new administration is expected to take a less restrictive attitude toward the supply of nuclear fuel and technology to potential nuclear weapons states, and is believed anxious to lift some of the restrictions imposed by Congress in the 1978 Nuclear Non-Proliferation Act. But it is unlikely that such changes could be legislated quickly and there is doubt that they can be obtained.

In agruing for continued shipments of fuel last fall, the Carter administration said U.S. restrictions on the nuclear fuel supplied in the past to India might be voided by failure to provide additional uranium as required under the 1963 pact. Now, however, the Reagan administration is arguing that these safeguards continue indefinitely despite termination of the pact. India rejects this view.

The future of the Tarapur fuel is of major importance. According to UPI, an Indian government scientist said in New Delhi yesterday that enough spent fuel has been accumulated at Tarapur to produce the raw material for nearly 200 explosions of the size of the 1974 atomic test.