Britain's trade minister said yesterday that attacks on other nation's unfair practices are unlikely to ease the United States' massive trade deficit.

Taking aim at a cornerstone of President Reagan's aggressive new trade policy and the rationale behind many congressional bills, Leon Brittan told a National Association of Manufacturers' luncheon that unfair practices by other countries contribute at most about $10 billion of this year's estimated $150 billion trade deficit.

He added, moreover, that other countries see many U.S. restrictions -- including federal and state "buy America" laws -- as unfair trade practices aimed at their exports to the United States.

"Unfair practices are not to be found uniquely on one side of the Atlantic," said Brittan in his first visit to the United States since taking up the trade portfolio in Prime Minister Margaret Thatcher's government last month.

"In matters of trade between the United States and the European Community," Brittan continued, "the field is not tilted in one direction . . . . but bumpy," and "needs to be leveled by negotiations and agreement and not by one team alone."

Brittan is one of a number of high-ranking foreign officials, drawn to the United States this fall by a congressional brushfire of support for tough trade laws, who have delivered lectures on the evils of protectionism. These leaders included Singapore Prime Minister Lee Kwan Yew, who took his attack on protectionism to a joint session of Congress the day the House was scheduled to approve sharp cuts in textile imports, and Japanese Prime Minister Yasuhiro Nakasone, who devoted most of his speech to the 40th anniversary session of the United Nations General Assembly to trade.

On Capitol Hill, meanwhile, Sen. Gary Hart (D-Colo.) offered legislation yesterday that he said would ease the trade deficit by improving the international competitiveness of American industries.