WASHINGTON (AP) _ A member of British Prime Minister Margaret Thatcher's government attacked part of the new U.S. trade law on Tuesday as protectionism ''masquerading as a tool to open up other markets.''

Trade Secretary Lord Young attacked a provision known as ''super-301,'' signed into law by former President Reagan in August. It threatens retaliation against countries which run persistent trade surpluses with the United States through what are judged to be unfair means.

''I do not believe that problems in world trade can be a matter for forced settlements, negotiated according to a timetable set by one party's legislature with a threat of unilateral trade action hanging over the other party's head,'' Young said in a speech at the National Press Club.

Young said he criticized the provision in visits with U.S. officials. On Tuesday he saw Carla Hills, the U.S. trade representative. He has also seen Commerce Secretary Robert Mosbacher.

Later Tuesday, in a speech in New York, Young said Americans need not worry that they will be excluded as the European Economic Community moves toward creating a single, borderless market by the end of 1992.

''It simply is not in our own interest to adopt protectionism,'' he told the United States Council for International Business. ''If we cannot compete, we will not survive.''

With its emphasis on deregulation, Young said, ''1992 is in many ways about the Thatcherization of Europe.''

In Washington, Young was critical of a bill that did not become law, which would have required registration of foreign investment in U.S. firms. He said it would have put extra conditions on foreign investment.

Britain is the largest foreign investor in the United States and vice versa, he noted.