Nothing so well illustrates the Reagan administration's inattention to the importance of foreign economic policy as its decision to move Bill Brock from his position as the U.S. trade representative to be secretary of labor.

"It came as a real shock," says a European diplomat. "Here we are trying to get ready for a new round of multilateral trade negotiations, and suddenly Brock -- the key player, the man who has given it momentum -- is gone."

President Reagan's fellow heads-of- government have frequently complained that the United States often acts without due regard to the effect its policies, programs and decisions have on its partners. By abruptly taking Brock out of his delicate negotiating role at a critical time, Reagan has reinforced this feeling.

The question of a new trade round to lower nontariff barriers in the important areas of telecommunications and other services -- as well as merchandise -- is scheduled for discussion at the Economic Summit in Bonn in early May. The hope had been to get it under way later this year or early in 1986.

"We have been preoccupied with this ever since we first heard that Brock was going to the Labor Department," said another foreign observer. "Does it mean that the president doesn't really care about a new round? Or does it mean that he didn't understand how important Brock has been in the process?"

The reality is that the White House seemed to have only one thing on its mind: the need to build new relationships with the labor movement, cut off from access to the administration during the entire tenure of Ray Donovan. Chief of Staff Donald Regan knew that Brock, a consummate politician, could rebuild bridges to AFL-CIO President Lane Kirkland and others.

Brock earnestly believes not only that he must re closed to the labor unions, but also that the administration needs the input of advice from labor and other groups it has tended to ignore.

But solving its labor/political problem by moving Brock to labor creates another one on the trade/diplomatic relationship side -- one that won't be solved easily, no matter who moves into the job. Uniquely, Brock has played a global role on trade issues: the prestige he had built up will take time for anyone -- no matter how experienced -- to duplicate.

Brock's departure also raises the question of the future of the office of U.S. trade representative. It should be remembered that the USTR under Brock held its own in a "turf" war with Secretary of Commerce Malcolm Baldrige's crew with little active White House support. This is a tribute to Brock's skills, including his negotiating ability -- as a former representative and senator -- on Capitol Hill.

Baldrge reportedly came to town with a promise from former White House aide Ed Meese that he would have the lead role on trade. But he has been forced to share the responsibility with Brock, and his proposal to swallow the USTR office into a super-trade agency called the Department of Trade and Industry has failed until now.

Brock has played a major role in cajoling the Japanese into providing greater access to their markets and is viewed in Japan as being more openminded than either Baldrige or Baldrige's chief deputy, Lionel Olmer. It was Brock, also, who led the fight against extension of Japanese auto quotas for a fifth year and who defused some of the most protectionist overtones in legislation passed by the last Congress.

Brock was by no means a purist: For example, in pursuing the art of the possible, he gave way to pressures last year from the American steel industry. He persuaded the president to reject a protectionist solution recommended by the International Trade Commission, but to negotiate severe import restraints on a "voluntary" basis with a number of countries. Thus, the administration's record, under Brock's USTR regime, was by no means wholly free trade.

But it was probably the best that was do-able in a world fast sinking into the mode of trade restraints. "In the last several months," says an international trade expert, "Brock was the most credible person in Washington on these issues. You got confused messages talking to anyone else. On the other hand, Brock was perceived as the one person who was saying the right things to his own government and to ours."

Unless an equally strong open-trade advocate is found to succeed Brock, things don't bode well for smooth trade negotiations or for USTR -- and it was for these reasons that Brock was not anxious to make the move, and did so only under strong pressure from the president himself.