A panel of three federal judges in Idaho has declared unconstitutional the 1970 law authorizing the Labor Department to conduct occupational safety and health inspections without a warrant.

The department fears the decision could strip it of all occupational safety and health enforcement power, and plans to appeal.

"We consider the right of inspection to be absolutely vital," Morton Corn, the assistant labor secretary who heads the Occupational Safety and Health Administration, said yesterday. "We will take whatever legal steps are necessary to vindicate that right."

The Idaho ruling, handed down last week, involved an electrical contractor, F. G. Barlow of Pocatello, who refused to let a federal inspector go through his place of business.

He rested his refusal on the Fourth Amendment to the Constitution, which forbids "unreasonable searches and seizures" and says search warrants should only be issued where there is probable cause to suspect a violation of the law.

The 1970 Occupational Safety and Health Act not only authorizes unannounced inspections of workplaces but prohibits advance warnings that inspections are about to occur. It makes no mention of search warrants, and federal inspectors do not obtain them.

In a Texas case earlier last year a three-judge panel said in effect that this failure to mention warrants must have been inadvertent, and ruled that safety inspectors ought to take warrants out in the future. That case is on appeal.

In the Idaho ruling, however, the panel simply said the law is unconstitutional, and left any repairs to Congress.

The Idaho ruling thus left OSHA without any authority to make inspections; the whole authorizing section was struck down as defective.

The Idaho judges then issued an injunction forbidding furture inspections of Barlow's place of business and possibly - the panel's meaning is not completely clear - forbidding all further OSHA inspections nationwide.

OSHA lawyers said they would appeal the constitutional finding the seek a stay of the injunction. An appeal would take the issue directly to the Supreme Court, which in the past has upheld some warrantless government health and safety inspections and struck down others.