A federal judge in Washington ruled on Monday that the bulk collection of Americans’ telephone records by the National Security Agency is likely to violate the US constitution, in the most significant legal setback for the agency since the publication of the first surveillance disclosures by the whistleblower Edward Snowden.

Judge Richard Leon declared that the mass collection of metadata probably violates the fourth amendment, which prohibits unreasonable searches and seizures, and was “almost Orwellian” in its scope. In a judgment replete with literary swipes against the NSA, he said James Madison, the architect of the US constitution, would be “aghast” at the scope of the agency’s collection of Americans’ communications data.

The ruling, by the US district court for the District of Columbia, is a blow to the Obama administration, and sets up a legal battle that will drag on for months, almost certainly destined to end up in the supreme court. It was welcomed by campaigners pressing to rein in the NSA, and by Snowden, who issued a rare public statement saying it had vindicated his disclosures. It is also likely to influence other legal challenges to the NSA, currently working their way through federal courts.

The case was brought by Larry Klayman, a conservative lawyer, and Charles Strange, father of a cryptologist killed in Afghanistan when his helicopter was shot down in 2011. His son worked for the NSA and carried out support work for Navy Seal Team Six, the elite force that killed Osama bin Laden.

In Monday’s ruling, the judge concluded that the pair’s constitutional challenge was likely to be successful. In what was the only comfort to the NSA in a stinging judgment, Leon put the ruling on hold, pending an appeal by the government.

Leon expressed doubt about the central rationale for the program cited by the NSA: that it is necessary for preventing terrorist attacks. “The government does not cite a single case in which analysis of the NSA’s bulk metadata collection actually stopped an imminent terrorist attack,” he wrote.

“Given the limited record before me at this point in the litigation – most notably, the utter lack of evidence that a terrorist attack has ever been prevented because searching the NSA database was faster than other investigative tactics – I have serious doubts about the efficacy of the metadata collection program as a means of conducting time-sensitive investigations in cases involving imminent threats of terrorism.”

Bravo. A judge with the courage to defend our Constitution – in the face of a President, Congress and most of our elected officials more than willing to defame the document that defines our history of freedom.