The Agriculture Department yesterday formally disputed charges by a Ralph Nader group that Reagan administration policies have undermined its meat inspection program and have threatened public health.

The department said it "has been enforcing and will continue to enforce vigorously the law of the land," noting that the rate of condemnation of meats by the government's inspectors generally has held steady since 1975.

It acknowledged that its Food Safety and Inspection Service has reduced "routine plant reviews," and that the number of inspectors has declined, but it said more efficient practices are protecting public health adequately.

The agency issued a 25-page report in response to a study released Jan. 31 by Nader's Center for the Study of Responsive Law. The report, written by Nader aide Kathleen Hughes, charged that relaxed standards and an increased work load on inspectors was opening the way for more contaminated food to reach consumers.

Delmer Jones, chairman of the National Joint Council of Food Inspection Locals, which represents about 6,000 federal employes, "endorsed and verified" the Nader charges.

The department acknowledged that "The need to target inspection resources in an era of federal financial austerity is producing morale problems between labor and management."