A US appeals court agreed Thursday to revisit a July ruling that threatened to undermine President Barack Obama's landmark healthcare reform law.

The US Court of Appeals for the District of Columbia announced that the full court will review a decision by a panel of three of its judges which several weeks ago disallowed federal subsidies in most US states under Obama's signature Affordable Care Act law.

By a vote of two to one, the panel said in July that the wording of the law authorizes federal subsidies for only state-run health care "exchanges"—insurance marketplaces where Americans can shop for health care coverage.

At present, most health care exchanges are run by and just 14 are operated by states.

The , if upheld, could void subsidies for millions of low-income and middle-income Americans, making their prohibitively expensive.

The appeals court scheduled a December 17 date to re-hear the case.

The decision to reconsider came amid unrelated revelations that the federal program's healthcare.gov website had been hacked.

News of the breach was released by the Department of Health and Human Services, which said hackers uploaded malicious software to the site, according to reports.

HHS, which told US media that no consumer information appears to have been stolen, said additional measures have been taken to shore up security in the wake of the attack.