WASHINGTON — The Supreme Court dealt a setback today to civil rights and privacy activists who oppose the Bush administration’s warrantless wiretapping program.

The justices, without comment, turned down an appeal from the American Civil Liberties Union to let it pursue a lawsuit against the program that began shortly after the September 11, 2001 terror attacks.

The action underscored the difficulty of mounting a challenge to the eavesdropping, which remains classified.

“It’s very disturbing that the president’s actions will go unremarked upon by the court,” the director of the ACLU’s national security project, Jameel Jaffer, said. “In our view, it shouldn’t be left to executive branch officials alone to determine the limits.”

The Terrorist Surveillance Program no longer exists, although the administration has maintained it was legal.

The ACLU sued on behalf of itself, other lawyers, reporters, and scholars, arguing that the program was illegal and that they had been forced to alter how they communicate with foreigners who were likely to have been targets of the wiretapping.

A federal judge in Detroit largely agreed, but the 6th U.S. Circuit Court of Appeals dismissed the suit, saying the plaintiffs could not prove their communications had been monitored and thus could not prove they had been harmed by the program.

The government has refused to turn over information about the closely guarded program that could reveal who has been under surveillance.

ACLU officials described the situation as a “Catch-22” because the government says the identities of people whose communications have been intercepted is secret. But only people who know they have been wiretapped can sue over the program.

A lawsuit filed by an Islamic charity met a similar fate. The 9th U.S. Circuit Court of Appeals last year ruled against the Oregon-based American arm of the Al-Haramain Islamic Foundation, concluding that a key piece of evidence is protected as a state secret.

In that case, the charity alleged the National Security Agency illegally listened to its calls. The charity had wanted to introduce as evidence a top-secret call log it received mistakenly from the Treasury Department.

A separate lawsuit against telecommunications companies that have cooperated with the government is pending in the San Francisco-based appeals court. A U.S. district court also is examining whether the warrantless surveillance of people in America violates the law that regulates the wiretapping of suspected terrorists and requires the approval of a secret court.

The administration announced in January 2007 that it would put intercepts of communications on American soil under the oversight of that court, the Foreign Intelligence Surveillance Court.

The ACLU, in urging the justices to consider its case, said that because the administration voluntarily ended the warrantless wiretapping, it could easily restart it.

The White House said the monitoring was necessary because the 1978 Foreign Intelligence Surveillance Act left dangerous gaps in the government’s eavesdropping authority.

Last August, Congress made temporary changes to FISA that made the warrantless wiretapping legal in some instances and also extended immunity from lawsuits to telecommunications companies that help with the intercepts.

Those changes expired over the weekend, amid disagreements between congressional Democrats and Mr. Bush over the immunity issue.

Existing wiretaps can continue and any new surveillance the government wants to institute has to follow the FISA rules, which could require court warrants.

The case is ACLU v. NSA, 07-468.