Federal authorities have always made it difficult to bring a legal challenge against the government’s warrantless wiretapping enterprise that was set up by the Bush administration in the years after the Sept. 11, 2001, attacks. Because the wiretaps were secret, no one could know for certain if they were being tapped, so the government urged judges to throw out lawsuits for lack of proof of real harm.

That strategy was halted on Monday when a federal appeals court said that civil liberties and journalism groups challenging an eavesdropping law could pursue a suit trying to get the government’s wiretapping declared illegal. In an important ruling, the United States Court of Appeals for the Second Circuit reinstated a lawsuit that a federal district judge had thrown out in 2009.

The new decision might lead to a significant — and far too long delayed — legal review of the statute.

The law in question, passed in 2008, amended the Foreign Intelligence Surveillance Act. It essentially legalized retroactively President George W. Bush’s outlaw program of wiretapping certain terror suspects without a warrant. It also immunized telephone companies that cooperated in the program.