A federal judge has issued a landmark ruling that finds BP to be primarily responsible for the 2010 oil spill in the Gulf of Mexico, finding that they showed reckless negligence while two other companies showed lower levels of negligence and have lower liabilities for the outcome and for paying for the cleanup.

U.S. District Judge Carl Barbier held a trial without a jury over who was at fault for the catastrophe, which killed 11 people and spewed oil for almost three months into waters that touch the shores of five states.

“BP has long maintained that it was merely negligent,” said David Uhlmann, former head of the Justice Department’s environmental crimes division. He said Barbier “soundly rejected” BP’s arguments that others were equally responsible, holding “that its employees took risks that led to the largest environmental disaster in U.S. history.”

The case also included Transocean Ltd. and Halliburton Co., though the judge didn’t find them as responsible for the spill as BP. Barbier wrote in his decision today in New Orleans federal court that BP was “reckless,” while Transocean and Halliburton were negligent. He apportioned fault at 67 percent for BP, 30 percent for Transocean and 3 percent for Halliburton.

U.K.-based BP, which may face fines of as much as $18 billion, closed down 5.9% to 455 pence in London trading…

The ruling marks a turning point in the legal morass surrounding the causes and impact of the disaster. Four years of debate and legal testimony have centered on who was at fault and how much blame each company should carry.

BP Exploration & Production Inc. is “subject to enhanced penalties under the Clean Water Act” because the discharge of oil was the result of its gross negligence and willful misconduct, Barbier held. BP said it “strongly disagrees” with the decision and will challenge it before the U.S. Court of Appeals in New Orleans.