BP shares have fallen after the US said it was suing the oil giant for alleged violations of federal safety laws over the Gulf of Mexico oil spill.
The lawsuit asks BP and and eight other firms be held liable without limitation for all clean-up and damage costs.
The Deepwater Horizon drilling rig explosion in April killed 11 workers and spilled millions of barrels of oil over several months.
BP's shares in London ended Thursday trading down 1.4%.
BBC business editor Robert Peston said that drop had taken the edge off a recent strong run in BP shares.
But he added: "Investors plainly believe that the nature of the Department of Justice's case against BP hasn't increased potential liabilities for the company in a fundamental way."
BP said that it would respond to the claims later, adding the action did not constitute "any finding of liability or any judicial finding that the allegations have merit".
The oil leak became the worst environmental disaster in US history.
And BP has set aside $39.9bn (£25bn) to cover the costs stemming from the disaster.
But our business editor said that if BP were found to be grossly negligent, the costs it faced could rise significantly.
It could potentially add almost $16bn to the penalties BP would have to pay under the US Clean Water Act, he said.
And it would make it "perhaps impossible" for BP to recover costs it is incurring in the clean up and restitution from its co-owners of the Macondo Well, Anadarko and Mitsui, he added.
The lawsuit charges the companies under the US Clean Water Act and Oil Pollution Act.
US Attorney General Eric Holder said the complaint alleged that "violations of safety and operational regulations" caused the explosion on 20 April.
The companies named in the lawsuit are BP Exploration and Production Inc, Anadarko Exploration & Production LP, Anadarko Petroleum Corporation, MOEX Offshore 2007 LLC, Triton Asset Leasing GMBH, Transocean Holdings LLC, Transocean Offshore Deepwater Drilling Inc, Transocean Deepwater Inc and insurer QBE Underwriting Ltd/Lloyd's Syndicate 1036.
The key accusations are:
- Failing to take necessary precautions to keep the Macondo well under control in the period leading up to the 20 April explosion
- Failing to use the best available and safest drilling technology to monitor the well's conditions
- Failing to maintain continuous surveillance
- Failing to use and maintain equipment and material that were available and necessary to ensure the safety and protection of personnel, equipment, natural resources and the environment
"We intend to prove that these defendants are responsible for government removal costs, economic losses and environmental damages without limitation," Mr Holder said.
"As investigations continue, we will not hesitate to take whatever steps necessary to hold accountable those responsible for this spill."
BP said it would continue to co-operate with government inquiries and fulfil its commitments to clean up spilt oil in the Gulf.
Halliburton, the company that cemented the Macondo well, and Cameron International, which provided equipment for the well, was not targeted in the lawsuit.
Transocean disputed the charges brought by the government, saying that it should not be held liable for the actions of others.
"No drilling contractor has ever been held liable for discharges from a well under the Oil Pollution Act of 1990," the company said in a statement.
"The responsibility for hydrocarbons discharged from a well lies solely with its owner and operator."