RICHMOND, Va. — A federal judge today declared the foundation of President Barack Obama’s health care law unconstitutional, ruling that the government cannot require Americans to purchase insurance.

The case is expected to end up at the Supreme Court.

In his order, U.S. District Judge Henry E. Hudson said he will allow the law to remain in effect while appeals are heard, meaning there is unlikely to be any immediate impact on other provisions that have already taken effect. The insurance coverage mandate is not scheduled to begin until 2014.

Even so, Republicans in Congress celebrated the ruling as validation of the arguments they had made for months while the law was pending.

Hudson is the first federal judge to strike down a key part of the law, which had been upheld by fellow federal judges in Virginia and Michigan. Several other lawsuits have been dismissed and still others are pending, including one filed in Florida by 20 states.

White House health reform director Nancy-Ann DeParle said the administration is encouraged by the two other judges who have upheld the law. She said the Justice Department is reviewing Hudson’s ruling.