Nearly three out of four retired corporate executives interviewed for a Justice Department study said that "industry cannot regulate itself and government regulations are necessary to prevent unfair and unethical practices that affect workers, consumers and competitors."

The 64 executives, who had been middle managers for major corporations such as General Motors, Mobil, Lockheed, Du Pont, Bendix and Procter & Gamble, expressed strong support for antitrust laws and regulations to protect consumers and workers.

Most of those interviewed said that they would tend to report serious worker-safety violations to the government but that they would not routinely report price-fixing, illegal rebates or illegal foreign payments.

The findings are part of a 117-page report, "Corporate Ethics, Illegal Behavior and Government Regulation: Views of Middle Management," completed last year by Prof. Marshall B. Clinard and publicized by consumer advocate Ralph Nader.

At a news conference Friday, Nader said that the Justice Department did not call attention to the report when it was issued in January because it undermines the Reagan administration's support of increased deregulation.

Nader called the findings "somewhat surprising" because they appear to contradict top executives' assurances of corporate responsibility.

A Justice Department spokesman said that four copies of the report, funded by a $24,000 National Institute of Justice grant, have been available for public viewing since January at the National Criminal Justice Reference Service in Rockville.

Clinard, professor emeritus at the University of Wisconsin and author of 11 books, including several on corporate ethics and corporate crime, said he asked the NIJ not to print more copies because he wanted it published commercially to ensure wider distribution than that achieved by a government report "that comes out and then disappears."

Clinard conducted in-depth, confidential interviews with 64 retired middle-management executives, asking them to rate the ethics of their industries and corporations and to explain specific ethical practices.

The study's highlights include:

* Top management's position and actual behavior are largely responsible for unethical and illegal corporate behavior and set the ethical tone for corporate compliance with laws and regulations, according to the retired executives.

* Top management usually knows about corporate violations of law, before or after they occur, the executives said.

* Asked, "Why do you think that some large corporations are unethical and violate the law a great deal while others appear to be ethical and seldom violate the law?" 34 executives, more than half of those interviewed, said that the personal ambitions and ethics of top management are the chief explanation. Thirty-one executives said illegal behavior is caused by competition or greed.

* The executives contend that the pressures of the corporate world, including the emphasis on showing a profit and keeping down costs, lead to unethical behavior in corporations.

* They said almost unanimously that top executives should publicly condemn the unethical and illegal behavior in the corporate world.