A high-ranking Common Market official warned yesterday that if a proposed new round of multilateral trade negotations fails, "it could be fatal to the international trading system.

"Therefore, better no new round than a round that would fail," Willy de Clerq, European Community commissioner for external relations, told a press conference.

The European Community Tuesday declared its readiness to participate in a new round of trade talks, and proposed initiating the discussions in Brussels, the community's headquarters. President Reagan and Japanese Prime Minister Yasuhiro Nakasone have both called for a new round of talks later this year or early in 1986.

But De Clerq stressed the European view that it would be useless to begin new trade talks unless several other steps are taken. "We have indicated our readiness to participate in the launching of a new round, but not any new round," he said.

De Clerq said Europe wants assurances that Japan will take concrete steps to allow a volume of imports consistent with its status as "an economic giant."

De Clerq also said it is important that trade talks not be held in a vacuum. Ways must be found, he said, to encourage greater stability of exchange rates -- perhaps by broadening the concept of the European Monetary System to the rest of the world.

De Clerc endorsed the reported willingness of Treasury Secretary James A. Baker III to take "a new look" at possible reforms of the international monetary system.

"I find that encouraging," De Clerq said. "We will have to sit at the table and discuss [all these issues] whether we like it or not. We can't talk about trade without discussing exchange-rate movements, and we can't be blind to capital flows."

In its provisional call for a new round of multilateral talks, the Council of Ministers attached other suggestions for "parallel consideration" with lowering of trade barriers. To diminish the fear of failure, the European Community suggests halting or rolling back protectionist measures, and achieving a consensus with the less developed countries (LDCs) in advance of the talks by assuring them that their problems will get adequate attention.

"The participation of the LDCs is most important if we want to have a new round [of talks]," De Clerq said. "A new round after all is not the club of industrial nations, it includes the LDCs as well, if we want any results. And I am rather optimistic about it. There is a movement toward a new round in which there is maximum participation."

De Clerq's timetable calls for further discussion of how to get the new round of talks started at the OECD Ministerial meeting in Paris in April, followed by an endorsement of the talks at the Bonn economic summit in May. If all goes according to plan, the EC has suggested an informal meeting of senior officials of the General Agreement on Tariffs and Trade to draw up a broad agenda prior to the actual convening of a new round of talks in Brussels.

De Clerq, although prodded by reporters, did not make clear what is being demanded of Japan. Tuesday's formal communique issued in Brussels, calling for the new talks, said that despite previous trade discussions, "concessions to Japan have not produced like results, and in consequence, an imbalance of benefits currently exists between Japan and her principal partners."

De Clerq, at the press conference, spoke of Japan as "a special situation." In the past, he said, there have been "encouraging words" from Nakasone, "but we have the necessity of seeing results." Pressed to elaborate, De Clerq said that actual evidence of increased imports by Japan was not a "prerequisite" to European approval of a new round of trade talks.

But he added: "We want imports into Japan to match those of other industrial nations . . . The problem is for everyone. Either everybody else is wrong and Japan is right, or everybody else is right and Japan is wrong."

EC officials said Europe had a trade deficit of $10.8 billion with Japan last year. If Japan increases its imports, Europe could sell it agricultural goods, chemicals, cars, pharmaceuticals and capital goods.