A federal judge in New Jersey has ruled that the U.S. Army Corps of Engineers violated the law when it followed Reagan administration instructions to ignore a provision of a new law designed to promote competition in contracting.

The law, passed last year as part of the Deficit Reduction Act, authorized the General Accounting Office to delay the awarding of a government contract if a timely, legitimate protest had been filed by a losing bidder. The Justice Department, however, had told agencies to ignore that provision because it was unconstitutional. It said the GAO, as an arm of Congress, could not overrule executive branch agencies.

Judge Harold A. Ackerman's ruling came on a case involving Ameron Inc., a contractor seeking a water pipe cleaning contract at the U.S. Military Academy. Ameron went to court after the Army ignored its plea to the GAO to delay the award.

Ackerman, in his decision Wednesday, said the legal position of the administration "seems to be one designed to avoid judicial resolution of the lawfulness of agencies' conduct" in disobeying the law.

"The functions of the comptroller general GAO chief in ensuring that contracts are awarded in accordance with the law is . . . constitutional as the comptroller general is appointed by the executive and, therefore, may exercise law enforcement functions," he said.

The ruling pleased Rep. Jack Brooks (D-Tex.), who wrote the statute, and House lawyers who had filed a brief in support of the firm.

"It's a resounding vindication for the House and the statute," said Stephen R. Ross, general counsel to the House Clerk. "It's high time for the administration to instruct the federal agencies and federal officials to comply with the law . . . ."