The government lost an antitrust test case yesterday when the Supreme Court ruled that bank directors may also serve on the boards of competing corporations, like insurance companies.

The court, in a 5-to-3 decision written by Chief Justice Warren E. Burger, said federal antitrust laws exempt bankers from the general prohibition on interlocking directorates among competing businesses.

Had the court ruled the other way, thousands of such interlocks would have come undone. About 50 percent of the directors of life insurance companies also serve on bank boards. Similar interlocks between banks and businesses such as brokerage houses might also have been affected.

The government contended that industries involving money lending--banking and insurance, for example--are competitive industries and that the interlocking directorates violated provisions of the Clayton Act designed to prevent concentration among competitors.

Federal officials, aware of the exemption for banks in the law, had never attempted to enforce the 60-year-old law against bankers. In 1975, however, they brought actions against three banks and bank holding companies, including Bankamerica Corporation, and four mutual life insurance companies in an effort to test the scope of the exemption.

They argued that while the section of the law in question exempted interlocks between banks, it did not exclude interlocks between banks and non-banking businesses, such as insurance firms.

Burger, reversing the 9th U.S. Circuit Court of Appeals, said the government was wrong. Had Congress wished to prohibit such interlocks, he said, it "would not have presented any difficulty to have said so explicitly."

Ordinarily, he acknowledged, the court defers to interpretations of the law by government agencies. It might have done so in this case if the government consistently tried to enforce such a prohibition, he said. "But the government does not come to this case with a consistent history . . . On the contrary, for over 60 years, the government made no attempt, either by filing suit or by seeking voluntary resignations" to enforce it.

" . . . The business community directly affected and the enforcing agencies and the Congress have read this statute the same way for 60 years," Burger wrote. "While these views are not binding on this court, the weight of informed opinion over the years strongly supports" yesterday's ruling, he said.

Justice Byron R. White, joined by Justices William J. Brennan Jr. and Thurgood Marshall, dissented. The court, White wrote, has exempted "an entire species of interlocks from any regulation whatsoever, even though such interlocks undisputably may have serious anticompetitive consequences directly contrary to the policies of our antitrust laws. I am quite sure," White wrote, "that Congress intended no such result."

John S. Kingdon, a lawyer with the Washington firm of Howrey and Simon, which represented Prudential Insurance in the case, said the decision is particularly welcome to "smaller companies and banks.