A US judge has ruled BP was "grossly negligent" in the lead-up to the 2010 Deepwater Horizon oil spill in the Gulf of Mexico.
The ruling could potentially cost BP billions more in compensation payments.
The New Orleans judge Carl Barbier also found BP subcontractors Transocean and Halliburton "negligent".
The 2010 oil spill was the worst in US history, and BP has set aside $43bn (£26bn) to cover fines, legal settlements, and clean-up costs.
BP said in a statement that it "strongly disagrees" with the ruling and that it would appeal to a higher court.
"The law is clear that proving gross negligence is a very high bar that was not met in this case," said the firm.
Shares in BP fell 6% after the ruling was announced.
Judge Barbier said BP should shoulder 67% of the blame for the 2010 spill, with drilling rig owner Transocean responsible for 30% and cement firm Halliburton responsible for 3%.
He ruled that BP will be "subject to enhanced civil penalties" due to its "gross negligence" and "wilful misconduct".
The ruling could quadruple the civil penalties that BP must pay as a result of the spill to an estimated $18bn.
Under the US Clean Water Act, a ruling of negligence would have meant BP was liable to pay $1,100 per barrel of oil spilled; gross negligence increases the penalty to $4,300 per barrel.
In its most recent annual report, BP said it had set aside $3.5bn for this case - an indication the firm had expected a more lenient ruling.
BP emphasised in its statement that it planned to argue during penalty proceedings for the lesser penalty.
In 2012, BP agreed to accept criminal responsibility for the disaster and agreed to pay $4.5bn to the US government, thus settling its criminal liability in the spill.
Now, legal efforts have focused on the amount of civil penalties the firm must pay, both to businesses and individuals affected by the spill and to cover environmental clean-up costs.
Also in 2012, BP reached a $9.2bn civil settlement and agreed to put $20bn into a trust to pay to businesses and individuals.
However, legal proceedings have continued after BP said it had been forced to pay compensation to some businesses and individuals who were not directly affected.
It is not clear what impact Judge Barbier's ruling will have on the legal wrangling over that settlement fund.
A date to determine the total number of barrels of oil that were spilled in 2010 as well as a final civil penalty has not yet been announced.
US government experts have estimated a total of 4.2 million barrels spilled into the Gulf; BP has said the figure is closer to 2.45 million.