Image caption The healthcare reform has divided US public opinion

A federal judge in the US state of Virginia has ruled against a key part of the Obama administration's law on healthcare reform.

The decision by US District Judge Henry Hudson is the first finding against the law passed in March.

He backed the state of Virginia's argument that the law's requirement that Americans purchase healthcare or face a fine was unconstitutional.

Other lawsuits are pending, but the US Supreme Court will have the final word.

The judge wrote in a 42-page decision that the disputed provision was "neither within the letter nor the spirit of the Constitution".

But he declined to invalidate the entire law, in what correspondents say was a small victory for Barack Obama.

'Critical milestone'

The decision has been welcomed by Virginia Attorney General, Kenneth Cuccinelli, who filed the lawsuit.

"This won't be the final round, as this will ultimately be decided by the Supreme Court, but today is a critical milestone in the protection of the Constitution," he said in a statement after the ruling.

The US Justice Department expressed disappointment, but said it continued to believe the law was constitutional.

"There is clear and well-established legal precedent that Congress acted within its constitutional authority in passing this law and we are confident that we will ultimately prevail," said a Justice Department spokeswoman.

The law on healthcare reform is seen by some as one of Mr Obama's biggest achievements during his first two years in office, but it continues to divide public opinion.

It aims to extend health insurance to millions of Americans who lack it - partly by requiring the mostly young, healthy Americans who currently forgo insurance to purchase it.

Two judges have rejected other challenges to the law, including one in Virginia last month.