A federal judge on Thursday ruled in favour of Republican lawmakers in their lawsuit against the health care reform known as "Obamacare", saying that the US government is unconstitutionally using public funds to make access to health services less costly.
Judge Rosemary Collyer, who was appointed to the bench by Republican president George W. Bush, said the federal government has no authority to spend public funds without the specific approval of Congress.
The decision will remain on hold, however, and Obamacare will remain in place since the government will file an appeal.
Collyer ruled that the administration may not spend billions of dollars in federal funds to pay insurance companies to reimburse them for cuts that the law requires them to make to customers' out-of-pocket medical payments.
Republicans hailed the ruling as "a critical step" in defense of separation of powers, in the words of Kevin Brady, who heads the House Ways and Means Committee.
However, the administration, which says that the authorisation for the spending is contained within the 2010 Affordable Care Act, announced that it will immediately file an appeal.
"This suit represents the first time in our nation's history that Congress has been permitted to sue the executive branch over a disagreement about how to interpret a statute," White House spokesman Josh Earnest told reporters at his daily press conference.
Conservatives have filed a number of lawsuits against the Affordable Care Act over the past six years since Congress approved it over Republican opposition.
The law has helped provide health care insurance coverage to millions of Americans who previously had none.