British Trade Minister Paul Channon yesterday urged the western industrialized nations to press Japan at the economic summit in Tokyo next week to reveal specific targets for buying more foreign-manufactured products.

"They keep telling us they have taken short-term measures to open their market. We want to know what their plans are and how they see their market opening up," Channon said during a visit here. He told reporters he was reflecting the view of the 12-nation European Community.

He said the community sees this as "an urgent need" for the summit because of "the lack of propensity of the Japanese to import." If pressure is not put on Japan, Channon predicted a continued growth of Japan's trade surplus, which will lead to political problems in Europe, Canada and the United States.

If Japan does not get specific, Channon said, "We will look to our GATT rights," referring to the General Agreement on Tariffs and Trade, the international system that regulates world trade. He refused to elaborate on that tactic, saying it would appear as if he were trying to "blackmail" Japan into taking action.

Channon appeared to be distinguishing Europe's position from that of the Reagan administration, which has eased pressures on Japan to open its markets in favor of Prime Minister Yasuhiro Nakasone's long-term program. This plan, accepted as a "historic" opening by the president during Nakasone's visit here earlier this month, would transform the Japanese economy to increase domestic demand and create as big a desire to import as the country now has to export.

But Channon pointed out that the proposal has run into increased political attack in Japan, where it does not even have the support of Nakasone's own party, and would take years to show results in terms of Japan's trade surplus with the rest of the world.

"There is a strong feeling in Great Britain and Europe that Japan's market is not open to manufactured imports in the same way as our markets are open to them," he said.

Channon said Japan has announced a number of programs to open its markets but never has revealed what it expects them to achieve. Because it has not revealed its targets, there is no way to judge the success of the programs, he argued.

He judged Japan's market-opening measures so far as "a modest start" that "isn't even visible on the ground. When I see a large number of American cars on the streets of Tokyo I will believe there is a start."

Channon's comments reflect an increasing hard line that Western Europe has taken on Japan's trading practices and its growing trade surpluses. The EC and Great Britain in the past have urged the Reagan administration to take joint actions with them against Japan, but have been rebuffed.

The EC representative to a GATT group planning for meetings to set new rules for world trade has suggested making the Japanese surpluses a central element of the trade talks. EC Ambassador Tran Van Thin, in a March speech, complained of one country that, "like a black hole, sucked all the benefits from GATT," to the detriment of the other members. Without naming Japan, he cited specifics that left no doubt he was referring to that country.