The United States has formally asked Japan to give foreign companies a direct role in setting technical standards and testing new telecommunications equipment to be sold in Japan next year.

The move reflects growing U.S. concern that Japan will use control of standards and testing to indirectly limit sales of foreign equipment after the expected deregulation of its telecommunications service industry in April.

Japanese officials, however, contend no such limits are planned. "This will become clear very soon," Kiyoshi Mori, an official of the Ministry of Posts and Telecommunications, said today. "They will understand that there will not be any discrimination."

Some Japanese officials have contended that the standards question is being blown out of proportion by misunderstanding and mistrust in the United States.

Currently, the giant Nippon Telegraph and Telephone Public Corp. (NTT) holds a monopoly over domestic telecommunications services and is charged with testing foreign and Japanese equipment before sales are allowed.

However, legislation expected to pass the Japanese Diet, or national legislature, next week would end NTT's monopoly and throw the field open to competition. Certification of new equipment would pass to a new private industry association that is being formed.

The U.S. move on standards, asking for direct representation of U.S. companies on the new approval body, followed extensive discussions here last month between Japanese officials and Clyde Prestowitz, counselor to the U.S. secretary of commerce, and James Murphy, assistant U.S. trade representative.

The U.S. position was conveyed in a memorandum delivered by the U.S. Embassy here to Japanese government agencies Wednesday. The memo called for the new approval process for interconnect equipment provided by users, such as telephone receivers, to "represent a step forward in eliminating technical trade barriers and in simplifying Japanese regulatory procedures."

U.S. officials have said that the government appears to be planning to limit membership to Japanese, in which case, they say, foreign products might not get a fair evaluation.

The memo also called on the Japanese to accept test data supplied from abroad in evaluating foreign equipment. Requiring all testing to be done in Japan would add excessive time and cost to the approval process, the United States contends.

Currently, NTT buys about $3 billion of new equipment each year, most of it from Japanese companies. Under a 1980 agreement, foreign producers are guaranteed equal footing in the bidding and have been slowly increasing their sales.

Deregulation will give users rights to shop around for a wide variety of equipment, as did the break-up of AT&T in the United States. Foreign suppliers here hope it will open up major new opportunities for sales.

The standards debate is one sign that trade friction between the United States and Japan is on the rise now that President Reagan and Prime Minister Yasuhiro Nakasone have both finished successful re-election campaigns.

In a speech here earlier this week, Lionel Olmer, undersecretary of Commerce for international trade, called on Japan to make increased imports a national goal. The U.S. trade deficit with Japan is expected to reach $35 billion this year and grow further in 1985.

Reagan and Nakasone are scheduled to meet in California in January. Trade issues are expected to figure importantly in those discussions.