MIAMI — A federal judge in Orlando struck down a Florida law on Tuesday that required welfare recipients to undergo mandatory drug testing.

Judge Mary S. Scriven of United States District Court wrote in her decision that the state's testing requirement was unconstitutional. "The court finds there is no set of circumstances under which the warrantless, suspicionless drug testing at issue in this case could be constitutionally applied," she wrote. The ruling made permanent an earlier, temporary ban by the judge.

The requirement had been a signature legislation of Gov. Rick Scott, who argued that the drug testing was necessary to help protect taxpayers and families. Mr. Scott said Tuesday that the state would appeal the ruling.

"Any illegal drug use in a family is harmful and even abusive to a child," he said in a statement. "We should have a zero-tolerance policy for illegal drug use in families — especially those families who struggle to make ends meet and need welfare assistance to provide for their children."

The case was also being closely watched by several other states, including Georgia, which passed similar legislation this year but found it dogged by legal challenges and statistics showing the requirement was ineffective. In Florida, less than 3 percent of those tested were found to have been using narcotics.

The decision stems from a 2011 suit filed by the American Civil Liberties Union and the Florida Justice Institute on behalf of a Central Florida resident, Luis W. Lebron, a Navy veteran who had filed for public assistance. Mr. Lebron, the single father of a 5-year-old who also provided care for his disabled mother, argued that it was unfair to require drug testing when no suspicion of drug abuse existed.

Judge Scriven agreed.

Howard Simon, executive director of the A.C.L.U. of Florida, said that "the courts are now signaling to politicians that they are not going to treat poor people as if they were exempt from constitutional rights."