A U.S. judge has decided that BP Plc was "grossly negligent" and "reckless" in the Gulf of Mexico oil spill four years ago, a ruling that could add nearly $18 billion in fines to more than $42 billion in charges the company took for the worst offshore environmental disaster in U.S. history.

BP said it would appeal Thursday's ruling by U.S. District Judge Carl Barbier in New Orleans, Louisiana, who held a trial without a jury last year to determine who was responsible for the April 20, 2010 rig explosion and spill that killed 11 workers and spewed oil for nearly three months onto the shorelines of several states.

Barbier ruled that BP was mostly at fault and that two other companies in the case, Transocean Ltd. and Halliburton, were not as much to blame. The disaster struck when a surge of methane gas known to rig hands as a "kick" sparked an explosion aboard the Deepwater Horizon rig as it was drilling the mile-deep Macondo 252 well off Louisiana.

Barbier has yet to assign damages from the spill under the federal Clean Water Act or say how many barrels spilled, but David Uhlmann, a University of Michigan law professor and former chief of the Justice Department's environmental crimes section, said the ruling "dramatically increases" BP's liability for civil penalties under the act.