NEW ORLEANS — BP could be looking at close to $18 billion in additional fines over the nation’s worst offshore oil spill after a federal judge ruled Thursday that the company acted with “gross negligence” in the 2010 Gulf of Mexico disaster.

U.S. District Judge Carl Barbier concluded that the London-based oil giant showed a “conscious disregard of known risks” during the drilling operation and bears most of the responsibility for the blowout that killed 11 rig workers and spewed millions of gallons of oil over three months.

In the next stage of the case, set to begin in January, the judge will decide precisely how much BP must pay.

Under the federal Clean Water Act, a polluter can be forced to pay a maximum of $1,100 in civil fines per barrel of spilled oil, or up to $4,300 per barrel if the company is found grossly negligent. Barbier’s finding exposes BP to the much higher amount.

Even as the oil giant vowed to appeal, BP stock fell $2.82, or nearly 6 percent, to $44.89, reducing the company’s market value by almost $9 billion.