Western defense experts urged European governments today to approve plans to deploy in Western Europe long-range tactical nuclear weapons capable of striking the Soviet Union.

Modernizing NATO's tactical nuclear force in this way would help to restore the credibility of the alliance's nuclear deterrence and demonstrate NATO solidarity, the experts said.

A working group will submit proposals along these lines next month, in time for North Atlantic Treaty Organization members to make a decision in December. The recommendations are likely to include the basing of Cruise missiles in Britain and Pershing 2 missiles in West Germany and at least one other continental European country, probably Belgium.

The appeals for support of the proposal were made at the close of a three-day session on NATO's future that brought together more than 100 Western defense experts. The conference was sponsored by the Georgetown University Center for Strategic and International Studies, a generally conservative research organization in Washington, and by the Atlantic Institute, a Paris-based group with ties to NATO experts.

The speakers urging NATO governments to accept the U.S.-made weapons included Joseph Luns, the NATO secretary-general; Henry Kissinger; former U.S. secretary of state; Henri Simonet, the Belgian foreign minister; and Gen. Alexander Haig, former NATO commander.

Kissinger said a rapid improvement in NATO's tactical nuclear force was necessary to help restore a balance of power with the Soviet Union, particularly during a period in the 1980s when, he said, the strategic nuclear balance will tip the Soviets' favor.

About the grim picture of Western vulnerability he painted in a speech here Saturday, Kissinger said he had not intended to cast doubts on the U.S. commitment to defend Western Europe with nuclear weapons. "The defense of Europe and the United States are indissoluble," he said.

Kissinger urged that NATO countries agree to deploy the new generation of missiles before entering talks with the Soviet Union on reducing Soviet and Western nuclear arms in Europe, perhaps through a third Strategic Arms Limitation Treaty.

Influential political factions in West Germany, Belgium and the Netherlands want NATO to propose disarmament talks to the Soviet Union before making the decision this year to deploy the weapons, which will not be ready until 1983.

Kissinger said, however, that this approach would allow the Soviet Union to influence the deployment of the weapons and perhaps delay their manufacture indefinitely.

West Germany has said it will base the new weapons on its soil only if another continental European ally also accepts them. Belgium is considered the most likely candidate.

Outlining NATO's potential problems in the next decade, Haig, who was NATO commander until the end of June, cited these concerns: a new sense of limits on U.S. power, the need for prompt action to restore the nuclear balance in Europe, and the lack of any clear military strategy on how to use these new weapons in an environment of overall Soviet superiority.

Many European participants here sounded less than confident that their citizens will support increased defense spending or a quick commitment to updating nuclear weapons in Europe.

Kissinger said Americans are "increasingly fed up with seeing the United States always on the defense." Mahy other conference participants also detected "a new mood of U.S. assetiveness," according to Christoph Bertram, one of the moderators of the conference.