BP's share price dived more than five per cent in late afternoon trading, after a US district court ruling made it liable for a fine of up to $18bn (£11bn) in relation to the Gulf of Mexico disaster in 2010.
The court found BP acted with "gross negligence" and "wilful misconduct" when the Deepwater Horizon well blew up, killing 11 people in 2010. The well took months to plug, with a US government estimate suggesting 4.2 million barrels of oil leaked into the ocean.
The court decided BP should be subject to the highest level of penalties under the US' Clean Water Act, which means it will pay up to $4,300 per barrel spilled, a total of $18bn. That represents a much larger sum than the $3.5bn the company had set aside to pay for Clean Water Act penalties, suggesting profits - which in 2013 were $13.4bn - will take a severe hit.
The judge also ruled BP was 67 per cent responsible for the disaster, while the other companies involved - Transocean and Halliburton - were 30 per cent and three per cent responsible respectively.
Earlier this week, Halliburton finally settled its lawsuits relating to the disaster, setting aside $1.1bn to pay property owners and fishing businesses.