(Reuters) - BP Plc. and Anadarko Petroleum Corp. narrowly failed to persuade a U.S. appeals court to reconsider its 2014 ruling that they could face civil fines under federal pollution laws over the 2010 Gulf of Mexico oil spill.

By a 7-6 vote, the 5th U.S. Circuit Court of Appeals let stand a three-judge panel's decision to uphold a 2012 ruling from U.S. District Judge Carl Barbier in New Orleans, in which he said the companies could face Clean Water Act penalties.

Barbier is scheduled on Jan. 20 to begin a non-jury trial to determine pollution fines. BP is appealing his Sept. 4 ruling that it was grossly negligent in causing the spill, exposing the London-based company to roughly $18 billion (12 billion pounds) of potential fines.

BP and Anadarko had owned a respective 65 percent and 25 percent of the Macondo well, which blew out following the April 20, 2010, explosion of the Deepwater Horizon drilling rig.

They said they should not face fines because the discharge that culminated in the largest U.S. offshore oil spill was the result of a broken riser under the control of Transocean Ltd, which owned the rig.

The three-judge panel ruled against BP and Anadarko last June 4, and issued a separate ruling five months later that the companies said caused confusion, further justifying a rehearing.

An outside spokeswoman for BP declined to comment. Anadarko spokesman John Christiansen also declined to comment.

Writing for the dissenting judges, Circuit Judge Edith Brown Clement said on Friday the panel misinterpreted the Clean Water Act, and misapplied its own standard in assessing what happened.

She said denial of a rehearing "ensures that our precedent concerning liability for oil spills under the Clean Water Act remains unclear."