A key provision in the Obama administration's landmark health care reform requiring all Americans to have medical insurance was unconstitutional, a judge has ruled.
It was the first major legal blow to President Barack Obama concerning the radical overhaul of the nation's health care system, which he has made a cornerstone of his administration.
The ruling by Judge Henry Hudson of the Eastern District Court in Richmond, Virginia, found that the new law's mandate that Americans must buy insurance or pay a fine goes beyond federal authority and violates the Commerce Clause, a key component of the US Constitution.
"Neither the Supreme Court nor any federal circuit of appeals has extended Commerce clause powers to compel an individual to involuntarily enter the stream of commerce by purchasing a commodity in the private market," Hudson wrote in his opinion.
Hudson, who was appointed by former president George W. Bush, ruled the provision "would invite unbridled exercise of federal police powers."
The legal challenge had been brought by Virginia's Republican attorney general, Ken Cuccinelli, an outspoken opponent of the bill.
But White House spokesman Robert Gibbs said: "The administration argued on the other side of this case and disagrees with the ruling.
"We're confident that it is constitutional."
The bill -- Obama's signature but controversial achievement during his first two years in the White House -- was signed into law in March.
But various US states have vowed to fight the law, which only passed after months of bitter wrangling in Congress, and have mounted legal challenges against it with about 20 suits currently before the courts.
Most experts say the fate of health care reform -- aimed at ensuring that some millions Americans who lack insurance are covered -- will likely be eventually determined by the US Supreme Court.
Hudson suggested as much in his ruling as well.
"This case... turns on atypical and uncharted applications of constitutional law interwoven with subtle political undercurrents," he wrote.
"The outcome of this case has significant public policy implications. And the final word will undoubtedly reside with a higher court."
Nancy-Ann DeParle, director of the White House office on health reform, played down the ruling, also pointing to the other cases pending on the constitutionality of the law.
"This is one of 20 and we have already prevailed in two others," she told CNN. "We believe the law is constitutional.
"We believe it is constitutional to say that everybody needs to be in the system, that everybody needs to have health insurance if they can afford it, if they can't they get help doing it."
She added: "The lawyers at the justice department they will be making decisions, and making a recommendation of how we move forward."
Republicans have vowed to try to roll back the reform when they formally take control of the House of Representatives in January, after making major gains in the November elections.
Leading Republican John Boehner, who is set to replace Nancy Pelosi as speaker, said: "Today?s decision is an encouraging sign for families and small business owners who have revolted against President Obama?s job-killing health care law and called for its repeal."
Rolling back the sweeping measure was a rallying cry for US conservatives, especially the "Tea Party" movement that helped power massive Republican gains in the November elections.