BP shares slumped by 6% after a US judge ruled that the oil giant was "grossly negligent" in the run-up to the Gulf of Mexico disaster.
The ruling, which is being contested by the company, paves the way for billions of dollars in additional civil penalties under the Clean Water Act.
Louisiana district judge Carl Barbier said BP bore 67% of the blame for the Macondo well disaster which killed 11 workers in April 2010.
BP said it will immediately appeal to the US Court of Appeals for the Fifth Circuit.
It said: " BP believes that the finding that it was grossly negligent with respect to the accident and that its activities at the Macondo well amounted to wilful misconduct is not supported by the evidence at trial.
"The law is clear that proving gross negligence is a very high bar that was not met in this case. BP believes that an impartial view of the record does not support the erroneous conclusion reached by the District Court."
The court is due to hold additional proceedings in January to consider the factors in determining a penalty under the Clean Water Act.
The statutory maximum penalty is 1,100 US dollars per barrel where the court finds simple negligence and 4,300 dollars per barrel US dollars where the court finds gross negligence or wilful misconduct.
In July, BP put the current cost of the disaster, which includes compensation and criminal fines, at 43 billion US dollars (£25.3 billion).
District Judge Barbier said in a 153-page ruling that BP made "profit-driven decisions" during the drilling of the well that led to the deadly blowout.
He wrote: "These instances of negligence, taken together, evince an extreme deviation from the standard of care and a conscious disregard of known risks."
The ruling means BP could face as much as 17.6 billion US dollars (£13.9 billion) in civil fines under the Clean Water Act, said David Uhlmann, a University of Michigan law professor and former chief of the Justice Department's environmental crimes section.
Mr Uhlman said: "It also repudiates BP's claims that it was merely negligent and will further damage BP's already badly-damaged reputation."
Judge Barbier was assigned to oversee most of the federal litigation resulting from the BP spill.
Last year, he presided over two phases of a trial for claims against BP and its contractors brought by the federal government, the five Gulf states and private lawyers representing businesses and residents.
The judge heard eight weeks of testimony without a jury for the trial's first phase, which was designed to identify the causes of the blowout of BP's Macondo well and assign percentages of fault to the companies involved in the drilling project.
Then the judge heard three weeks of testimony for the second phase, which focused on competing estimates of how much oil spilled into the Gulf and examined BP's efforts to seal the well.
Millions of gallons of crude gushed into the Gulf after the well blew and triggered an explosion on the Deepwater Horizon drilling rig, killing wildlife, staining beaches and polluting marshes.
BP ultimately sealed the well after several previous attempts had failed to stop it.
The oil giant pleaded guilty in January 2013 to manslaughter charges for the rig workers' deaths.
It also agreed to pay a record four billion US dollars (£3.2 billion) in penalties as part of its deal with the Justice Department, but the plea agreement did not resolve the federal government's civil claims against the firm.