The United States, reversing a longstanding policy, has dropped its opposition to sales by European countries of "defensive" arms to mainland China.

The U.S. decision was reached in principle by high administration officials last spring and conveyed to the Chinese during presidential assistant Zbigniew Brzezinski's trip to Peking last May, according to informed official sources.

The application of the policy has been the subject of further decision-making and some internal debates in the months since then, officials said.

As described by administration sources, the new guidelines toward arm sales to China consist of three major points:

The United States will not supply weapons to China or the Soviet Union.

The United States is opposed to any sales of "offensive" weapons that could threaten China's neighbors, especially Taiwan or the Soviet Union, or that would be a "destablizing" factor in Asia.

The United States will take no position on sales of non-threatening or "defensive" arms to China by NATO allies. In the past, the United States had opposed such sales, except for a few exceptions on a "case-by-case" basis during the Ford administration.

The formal American position is that it is neither encouraging nor discouraging the sales by European allies, a statement which has been made by high U.S. officials both to China and its sensitive arch-rival, the Soviet Union.

However, the practical effect of the policy shift is to give the green light to moderate arms sales to Peking by removing previous American objections.

The U.S. stand was made known to France, Britain and other allies who had been approached in China's renewed quest for modern arms, but it was publicly acknowledged here until last Friday.

In response to a news conference question then about "essentially defensive weapons," Secretary of State Cyrus R. Vance said that, while it is "strong and unequivocal policy" of the United States not to sell weapons to China or Russia, "insofar as other nations are concerned, this is a matter which each of them must decide for itself."

One of the problems involved for the United States in the policy shift is how to handle clearance of the weapons sales by the interallied Coordinating Committee (COCOM), which was set up in 1949 by the United States and its allies to control the export of strategic items to communist countries. Arms of all kinds are on the list of items monitored by COCOM.

Current indications are that COCOM will be bypassed by an informal understanding that the selling nation need not apply for COCOM clearance of "non-threatening" weapons sales to China, and that the United States will not object to this omission.

A delicate U.S. legal issue involves the application of the 1951 Batte Act declaring that it is U.S. policy "to apply and embargo" on weapons sales to nations threatening American security. China has been included in this category in the past by executive branch decision.

The first implementation of the new policy, according to officials, occurred when Britain recently informed the United States of plans to sell diesel engines to China for use in a new class of coast guard vessels. The United States replied, after concluding that such a sale would not threaten Taiwan, that it has no position on the matter.

Currently under discussion or negotiation are more important sales to Peking or French antiaircraft and antitank missiles and helicopters, and British short-range, vertical takeoff jet fighters. Although the United States has not yet been officially informed, the general expectation is that it will take no position on these sales.

An advantage of the new policy, so far as U.S. officials is concerned, is that Washington will be taken out of the cross five between Moscow and the European allies regarding weapons sales.

A disadvantage, which has been voiced by the American arms industry and its Pentagon supporters, is that the United States is passing up a possibly lucrative new market and in effect awarding it to arms makers abroad.