BRUSSELS -- President Bush has proposed that the United States and its European allies modify three decades of defense doctrine and make nuclear arms "weapons of last resort" to ward off Soviet aggression in Western Europe, diplomatic sources have disclosed.

Bush's proposal to move away from the "flexible response" strategy -- which threatened a quick Western nuclear riposte to Soviet invasion -- is part of a wide-ranging U.S. blueprint for change in North Atlantic Treaty Organization doctrine and structure. The Bush statement, in effect a draft declaration, has been circulated to European leaders in advance of the alliance's summit in London on Thursday and Friday.

The proposed changes in NATO doctrine are, in part, an attempt to reassure and bolster Soviet President Mikhail Gorbachev, who is expected to come under attack from conservative critics at a Soviet party congress that opens in Moscow today. The Bush statement also endorses radical changes in the alliance's traditional "forward defense" deployment in West Germany, the sources said, which should also reassure the Soviets.

Under existing NATO doctrine, tactical nuclear weapons could in theory be used in the early days of a battle if Warsaw Pact conventional forces appeared to be gaining the upper hand. By making clear that the alliance would wait to use tactical nuclear weapons only as a last resort, NATO would in effect commit itself to no early first-use of nuclear weapons without formally renouncing the right to strike first.

The NATO summit also will consider changes in military hardware in response to the reduced threat from the Warsaw Pact. The Washington Post reported Sunday that the Bush administration has proposed to its allies the eventual withdrawal of the U.S. arsenal of nearly 1,400 nuclear-tipped artillery shells from Western Europe.

The U.S. proposal on nuclear-arms use is being closely coordinated with a similar, more detailed document being drawn up by NATO Secretary General Manfred Woerner and his staff. Secretary of State James A. Baker III is said to have taken particular care to get agreement from the West German government of Chancellor Helmut Kohl, which is working to bring a united Germany into NATO despite Soviet objections.

The only country that has replied coolly to the U.S. draft thus far is France, according to one authoritative source. French President Francois Mitterrand is unhappy with U.S. insistence that the summit address in detail in its final declaration the future institutions and role of the 35-nation Conference on Security and Coooperation in Europe (CSCE).

Mitterrand apparently does not differ greatly in substance with the American proposal for a small executive body for the pan-European organization. But he reportedly feels that it is not proper for NATO as an alliance to prescribe the shape of CSCE, which the Soviets hope will eventually replace both NATO and the Warsaw Pact as a common European security organization.

French-American differences over the future of NATO are the most pronounced within the 16-member alliance, diplomats said. A sharp exchange can be expected if the United States renews earlier suggestions that NATO should now redefine its mandate to include joint operations outside Europe, these diplomats said.

But Bush studiously did not mention these "out-of-area operations" in his blueprint. French officials expect Mitterrand to work for an outwardly smooth summit by subordinating his concern over CSCE if the United States does not push the issue of out-of-area operations.

Although he reportedly is also concerned that the evolving U.S. position on NATO nuclear strategy lacks clarity, Mitterrand will voice his reservations only tangentially, since France has not taken part in NATO nuclear planning or doctrine since Charles de Gaulle withdrew his nation from the alliance's military command in 1966.

U.S.-French arguments over nuclear strategy were a major factor in development of the "flexible response" doctrine by the Kennedy administration at the beginning of the 1960s. After arduous discussions with NATO, the United States persuaded the alliance to adopt it as policy in 1967.

Under the Eisenhower administration, the United States had threatened to reply to a Soviet attack on Europe with an all-out nuclear assault on the Soviet heartland. But de Gaulle and other Europeans argued that as the Soviets amassed their own nuclear intercontinental rockets and other delivery systems capable of dealing crippling blows to U.S. territory, that promise was losing its credibility.

The United States would not trade Chicago for Hamburg or Lille, de Gaulle maintained in justifying France's own independent nuclear force.

To move away from the all-or-nothing situation that the massive reprisal doctrine appeared to represent, the United States switched to "a ladder of escalation" philosophy, under which a variety of tactical nuclear warheads were deployed for use only in Europe and at any stage of the battle.

Throughout the Cold War, the United States repeatedly refused to rule out using nuclear weapons first if the Soviets used its numerical advantage in tanks, manpower and aircraft to launch a non-nuclear ground attack against Washington's European allies. In the preparations for the summit, the Bush administration has emphasized again that it will not agree to a declaration that would formally rule out first use.

But the blueprint circulated by Bush does acknowledge that Warsaw Pact conventional superiority is rapidly disappearing and that a new defense doctrine must be adopted now to reassure the Soviets and to show Western public opinion that the West is working for peace, according to a European official aware of the contents of the Bush letter.

The Soviets see flexible response as a "war-fighting strategy" intended to allow NATO to win each stage of a European battle. The Soviets claim that their own extensive theater and nuclear weapons are intended only as a deterrent force to keep the United States from attacking.

By shifting to a pledge to use nuclear weapons only as a "last resort," the United States hopes to keep the nuclear option while demonstrating an awareness that the likelihood of a Soviet ground attack has virtually disappeared, an allied official said.

The summit is also likely to endorse a gradual thinning and replacement of NATO's current "layer cake" defense along what is now West Germany's eastern frontier by separate U.S., British, Belgian, Dutch and West German army corps with much smaller, multinational units.

The allied armies were segmented and deployed forward along the frontier in eight corps of 55,000 men each to make sure that a broad Soviet attack involved as many alliance members as possible. One purpose of the proposed multinational units is to maintain that political symbolism of shared risk, European officials say.

"The basic message the West wants to get across to the Soviets is that we no longer look at them as adversaries but as partners in security," said one official familiar with the Bush message. The blueprint also asks the summit to endorse the president's call for negotiations on short-range nuclear missiles and adds a pledge to reopen immediately the Conventional Forces in Europe negotiations in Vienna to take up troop limitations once agreement is reached in the current talks on armor and other weapons.