ASHINGTON, March 2 - A contractual agreement between the government and one of its spies is unenforceable in federal court because of the "absolute secrecy" the government needs for its espionage activities, the Supreme Court ruled unanimously on Wednesday.

The decision, with an opinion by Chief Justice William H. Rehnquist, overturned a federal appeals court ruling that permitted a lawsuit by two former Soviet bloc spies to go forward in federal court.

The suit, charging a breach of a promise to provide lifetime financial support, had not gone to trial in federal district court in Seattle, where the former spies, a married couple from an unnamed country, filed it in 2000.

The government took the case to the Supreme Court last spring, arguing that even the need to respond to the allegations of such a lawsuit posed a threat to national security by requiring an acknowledgment that an espionage arrangement existed.

The government argued that the case, Tenet v. Doe, No. 03-1395, should have been governed by a rule the Supreme Court established in a post-Civil-War spy case called Totten v. United States, decided in 1876. In that case, the heirs of a spy who had entered into a contract with President Abraham Lincoln to infiltrate the Confederate lines for $200 a month filed a lawsuit contending that he had not received full payment. The Supreme Court ordered the suit dismissed.

As Chief Justice Rehnquist summarized that decision on Wednesday: "We thought it entirely incompatible with the nature of such a contract that a former spy could bring suit to enforce it." The reasons for the rule are still valid today, he said, and the precedent should apply even though the current lawsuit was framed not as a contract suit but as a complaint for a violation of constitutional due process.