Pat Choate, a prominent commentator on trade and a vocal critic of Japanese lobbying efforts, yesterday was forced out of his policy position in the Washington office of TRW Inc., a conglomerate with extensive Japanese commercial ties, friends said yesterday.

They quoted Choate as saying that TRW's Japanese customers, who do about $400 million in business a year with the firm, pressured TRW to get rid of Choate in reaction to his past criticism and his forthcoming book, "Agents of Influence," to be published in October. The book charges that Japan buys influence in Washington in an attempt to put its stamp on U.S. trade and economic policies.

Conflicting versions of the circumstances of his departure seem likely to add more fuel to the debate over Japan's economic rivalry with the United States.

Last year, friends said, a retired high official of the Japanese government, Matsudo Kuroda, talked to TRW Chairman Joseph P. Gorman about Choate.

"I never asked him {Gorman} to fire Choate," said Kuroda, former vice minister of the Ministry of International Trade and Industry and now an official with Mitsubishi Corp., after being reached in Tokyo last night. "I asked him if {Choate's books and articles} represented the opinion of TRW. I told him many people may consider that the opinion of TRW."

"TRW officials got very nervous because of all the attention the book was getting," said one friend. "Pat's being separated from TRW is a metaphor for the book," said another.

The friends did not want to be named and said they feared that publicity would hurt Choate's separation agreement, which includes an opportunity to work as a consultant for TRW.

TRW yesterday denied that Choate was forced out of his job and said that he resigned as vice president for policy analysis to pursue a full-time career as a writer and lecturer. "Dr. Choate has not been fired," said TRW spokesman Mike Johnson.

He issued a statement quoting TRW Chairman Gorman as saying, "We have long recognized Dr. Choate's unique capabilities and his unique contributions to TRW. We wish him the very best in his new endeavor."

Choate, reached in Santa Fe where he is on vacation, declined comment. The TRW statement quoted him as saying, "I thoroughly enjoyed my association with TRW and I look forward to embarking on the next phase of my career."

The bearded Choate, who as a boy picked cotton on his father's farm in Maypearl, Tex., had become a kind of Washington archetype: a policy junkie who combined a PhD in economics, a sharp mind, glib tongue and a skillful writing style to impact the capital's political debate. His major theme concerned strategies for strengthening American industry to make it more competitive in a world in which economic power had become more important than nuclear weapons.

He was nonpartisan, dealing with both Republicans and Democrats on Capitol Hill. And he seemed to be everywhere: appearing on television talk shows and think tank panels, testifying at congressional hearings and being quoted in numerous newspaper and magazine articles on the state of the American economy. He organized the Congressional Economic Leadership Institute, which held frequent luncheon meetings to deal with competitiveness issues. He was as likely to appear before the Democratic Leadership Council as the Republican House Wednesday Group -- both middle-of-the-road organizations for their parties.

He still worked full time for TRW, whose former chairman, Ruben F. Mettler, hired him to deal with national policy issues. His assignment was to focus on long-term public policy issues of particular importance to TRW, the company said, and its statement praised Choate for drawing "national attention to America's manufacturing competitiveness and the declining condition of our national infrastructure."

In recent years Choate had become a visible member of a group of American analysts advocating a new, tougher approach to Japan. A prominent public policy lobbyist himself, he became known particularly for his criticism of Japan's lobbying strategies, particularly its hiring of former high U.S. government officials to plead its cause with federal, state and local officials.

His book, which will be previewed in a Harvard Business Review article this fall, reportedly describes extravagant efforts by Japan to influence governmental and academic thinking, in the latter case through contributions to universities and think tanks.

Even before the book was out, its central theme has been attacked in publications such as the Wall Street Journal and the Economist.

The book's publisher, Alfred Knopf, is planning a large first printing of 50,000 copies. Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.), the 1988 Democratic vice presidential nominee, is preparing to hold hearings on Japanese influence in Washington, and staff aides said Choate is a likely witness.