By Daniel Gilbert 

BP PLC faces a maximum $13.7 billion in fines for the oil that gushed into the Gulf of Mexico in the 2010 Deepwater Horizon disaster, after a federal judge ruled on Thursday that the company is liable for just over 3 million barrels of spilled crude.

The decision by Judge Carl Barbier came days before BP is set to appear in his court for a related trial on how much it should pay per barrel. That tranche of the complex case, which begins on Tuesday, will determine the total fine under the U.S. Clean Water Act.

In September, Judge Barbier ruled that BP acted recklessly in the events leading up to the rig explosion and oil spill, exposing it to a maximum pollution penalty of $4,300 a barrel. BP has asked the per-barrel fine to be capped at $3,000.

The ruling on Thursday concluded that 4 million barrels of oil spilled into the Gulf, and that after taking into account the crude that was skimmed off the surface, BP is liable for a fine on 3.19 million barrels.

The company, however, wasn't "grossly negligent, willful, or wanton" its efforts to control the spill, Judge Barbier wrote.

BP had argued it should be liable for 2.45 million barrels, while federal prosecutors claimed a fine should be based on 4.2 million barrels.

The company didn't immediately respond to a request for comment.

BP has set aside $3.5 billion to pay for penalties under the Clean Water Act. It has already spent $43 billion on spill-related costs, including criminal and civil settlements and $14 billion for the Gulf cleanup.

The ruling also has implications for Anadarko Petroleum Corp., which owned 25% of the well drilled by the Deepwater Horizon rig. Anadarko's maximum liability is $3.5 billion based on the decision. The company has set aside $90 million for a potential fine. It didn't immediately respond to a request for comment.

Write to Daniel Gilbert at daniel.gilbert@wsj.com

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