The Supreme Court threw out a lower court decision yesterday that the Clinton administration had asserted could lead to the release of sensitive diplomatic communications and damage U.S. foreign relations.

By an 8 to 1 vote, the justices canceled a lower court ruling and dismissed a freedom-of-information case that was scheduled to be argued next week, after the Justice Department released the letter in question and said the matter was moot. The department told the court it recently had learned that the substance of the document, about a British woman's extradition to the United States, already had been disclosed.

Lawyers on the other side criticized the administration's action as a preemptive move to get rid of an unfavorable lower court decision.

The case revolved around a letter sought by Leslie R. Weatherhead, an attorney for Sally Croft, a follower of Indian guru Bhagwan Shree Rajneesh who was extradited to face charges of conspiring to kill a U.S. attorney in Oregon. Weatherhead sought a July 1994 letter sent by British officials to the Justice Department concerning the extradition. The department denied his request, saying the letter was exempt from the Freedom of Information Act based on national security.

The U.S. Court of Appeals for the 9th Circuit ruled, however, that the government failed to show how the letter's release could damage security interests. In its appeal, the Clinton administration argued that courts should be more deferential to executive branch claims that a disclosure would hurt foreign relations.

Then on Nov. 23, just two weeks before the case was to be heard, Solicitor General Seth P. Waxman said the letter had been released. He said that because a British consul had earlier given Weatherhead much of the substance of the Croft letter, it was no longer necessary to keep it secret. He asked that the 9th Circuit ruling be removed from the record, to prevent it from "chilling future confidential communications between the United States and . . . foreign governments."

Only Justice Antonin Scalia dissented, without explanation, in the case of United States v. Weatherhead.