The Supreme Court ruled yesterday that federal employes who are represented by a union at a grievance proceeding may not be afforded any more procedural safeguards than nonunion workers.

In a setback for the 1.2 million federal workers who belong to unions, the court held, 6 to 2, that the same standard of review -- that the agency committed a "harmful error" in disciplining an employe -- applies for overturning agency actions in both union and nonunion cases.

Under a 1978 law, federal employes may challenge a disciplinary action by appealing their agency's decision to the Merit Systems Protection Board (MSPB). If the employe is a member of a union, he may instead challenge the decision through the grievance and arbitration procedure provided by the union's collective-bargaining agreement.

The Supreme Court's ruling means that the same standard of review for overturning an agency's ruling will apply, whether the grievance is taken to an arbitrator or to the board.

The issue decided in yesterday's union case, the court said, was whether an arbitrator may overturn an agency's disciplinary action on the basis of a significant violation of the collective-bargaining agreement that is harmful only to the union.

Instead the court adopted a harmful error standard, which is used by the MSPB, requiring a showing of "substantial prejudice" to the employe's rights.

Writing for the court, Justice Harry A. Blackmun said Congress "clearly intended" arbitrators to apply the same substantive rules that the board does when they review a disciplinary action.

The case, Cornelius v. Nutt, arose in 1982 after two General Services Administration employes in Denver, Thomas Rogers and Robert Wilson Jr., were dismissed for falsifying records and for other charges in connection with drinking beer while on duty.

The two appealed to an arbitrator, complaining that they had not been notified of their right to have a union representative present during an agency investigation.

The arbitrator, Allison E. Nutt, found that Rogers and Wilson were guilty of misconduct, but reduced their penalty to suspension for two weeks without pay "because of the agency's pervasive failure to comply with the due process requirements of the collective-bargaining agreement."

That decision was upheld by the Court of Appeals for the Federal Circuit, which said that an arbitrator may consider significant violations of a collective-bargaining agreement that are "important to the union," even though the particular parties to the grievance may not have been affected.

Yesterday's ruling rejected the appellate decision, but Blackmun noted that it did not "prevent the union from obtaining a binding interpretation of a disputed provision of the collective-bargaining agreement or from enforcing agency compliance with that decision."

In a dissent, Justice Thurgood Marshall, joined by Justice William J. Brennan, said the court's decision "undermines the clear congressional intent" of encouraging collective-bargaining agreements.

Justice Lewis F. Powell did not participate in the case.

In other action yesterday, the court agreed to decide whether unions representing federal employes can challenge government contracts to private enterprises.

The U.S. Court of Appeals here rejected last year a claim by the Equal Employment Opportunity Commission that it should be free to contract with private firms for goods and services without challenge by an employe union.