Soviet leader Leonid I. Brezhnev today called for a full moratorium on all nuclear explosions, including those for peaceful purposes, reversing his country's long-held position on this matter and apparently removing a major impediment to a comprehensive test ban treaty between the United States, Britain and the Soviet Union.

American negotiators at Geneva, where the three nuclear powers are now bargaining for a complete ban on all nuclear tests, have long maintained that such a ban must include "peaceful" explosions as well as tests of military warheads. The reason is U.S. fear that the Soviets or any other signatory could mask a military test as a peaceful explosion.

The proposal by the Soviet Union's president now apparently swings Moscow into the same position with possible major impact on the slow-moving Geneva talks.The Soviets had consistenly taken the position that peaceful explosions should be allowed under any total ban treaty, in part on the grounds that they are needed for development projects in the vast untapped regions of this nation's interior.

Brezhnev made the policy change known toward the end of a 90-minute speech before the Supreme Soviet the nation's Parliament, and other Soviet Communist Party leaders in a session marking the 60th anniversary of the Bolsbevik revolution.

"We state that we are prepared to reach agreement on a moratorium covering nuclear explosions for peaceful purposes along with a ban on all nuclear weapons tests for a definite period," he said. "We trust that this important step on the part of the U.S.S.R. is properly appreciated by our partners at the negotiations and that the road will thus be cleared to concluding a treaty long awaited by the people." Brezhnev said a comprehensive test ban has a direct bearing on the task or reducing the danger of nuclear war, so that no such tests are conducted underground as well as in the atmosphere, in outer space and under water. We want ot achieve progress in the negotiations on this matter and bring them to a successful conclusion."

At present, the Geneva negotiations have been slowed by tow problems have paractivally been climinated with regard to tests of nuclear devices everywhere except underground. There some nuclear experts contend, it is at least theoretically possible to explore a nuclear device in such a way as to mask it as a natural earthquake or other underground phenomenon.

Nuclear weapons development depends heavily on testing to prove out the complex calculations of physicists andd designers regarding such vital properties as weight, size and yield of a warhead. The elimination of tests through a treaty world tend to slow down the arms race fro that reason leaving weapons advances on paper instead of converting them to hard ware where they immediately change the balance of terror between the superpowers.

Informed Western diplomats called Brezhnev's offer "A useful step" toward a total nuclear test ban.

This looks like an important narrowing of differences," said one source. "Such a step had not been expected and this looks like it could eliminate one main obstacle to the agreement." This source cautioned however that the United States must wait for the full details of Brezhnev's proposal to be laid on the bargaining table before it would be said with certainty that the Soviets have abandoned fully their earlier position.

Since testing is so vital to weapons development one source observed, Brezhnev's move indicates that the Soviets feel confident now that they have the technical capavity to ban tests and still maintain their weapons position with regard to the United States."

The Soviet proposal in one more in a series of abrupt improvements in Soviet-American relations that began sometime in September Soviet anger over President Carter's outspoken strategic arms policies and his gestures of support for political dissidents here began to melt with the U.S. reponse to a Soviet allegation that South Afirca was about to explode its first nuclear device in the Kalahari desert. The United States and Britain extracted a promise from South African Prime Minister John Vorster that his country would not test any nuclear device.

Since then the United States and the Soviet Union have issued identical statements of cooperation to achieve a new strategic arms limitation agreement, promised to maintain their strategic arsenals at present levels so long as the other nation does the same and are cooperating on reopening the Middle East peace conference which has been adjourned since a three-day session in Geneva in 1973.

A source here speculated that, if a complete test ban treaty can be negotiated at Geneva. President Carter's Congress that the verification question is settled.

Nuclear testing has gone on continously since the first U.S. blast in 1945. The United States and the Soviet Union achieved a partial test ban in 1963, eliminating surface and atmospheric explosions.

At present, the United States Soviet Union and Britain are pledged to test only under ground. France has not conducted any atmospheric tests in recent years. China, however, has continued atmosphere testing. The impact of radioactive fallout in even miniscule amounts is on the health of humans and their environment is unknown.

Brezhnev's speech also contained a second nuclear limiation proposal that "agreement be reached on a simultaneous half in the production of nuclear weapons by all states." One informed source called that "an obvious non-starter." The source charaterized it as "one more in the series of major weapons proposals that have no future and are used primarily for propaganda purposes."

As it turned out, Brezhnev forgot to deliver the proposal skipping over it in his speech although Tass, the official Soviet news agency duty reportes it. A spokesman said later that it should be considered part of the text of the "official" speech. Brezhnev who will be 71 next month has on occasion dropped sentences, paragraphs and pages from his addresses.

Today's speech also touched on detente, U.S. relations and Soviet relations with China. When he described China as having "ignored the economic laws of socialism . . . and joined the forces of reaction in the world arena," the new Chinese ambassodor Wang Yu-ping, staked out.

The Soviet communist Party leader said, "It is natural that we aattach great significance to relations with the U.S." While saying the "is much that divides our countries, he added that it is important not accentuate differences for tear of a buildup of distrust and hostility on dangerous of the world. Life itself require that considerations of long-term charachter prompted by a concern for peace be decisre in Soviet-American relations. This is the court we follow. And this what we expect in return.