BP is suing Transocean, the owner of the oil rig that exploded in the Gulf of Mexico last year, for $40bn (£24.37bn) in damages.
The British firm argues that security devices on the rig failed. This implies that Transocean are responsible for last year's oil spill, the largest environmental crisis the US has ever seen.
Separately, BP has also started legal action against Cameron International, the company that provided the oil rig's blowout, which was deemed faulty by a testing firm hired by the American government.
Both legal actions were launched in a Federal Court in the state of New Orleans -- a state heavily affected by the disaster -- on Wednesday 20 April, exactly a year after an explosion on the Deepwater Horizon killed 11 crew-members and triggered the oil spill. The date was the deadline for all parties involved to launch legal procedures.
BP's legal actions are intended to cover the firm's own liabilities, which come close to $40.9bn and could increase by tens of billions in government-imposed criminal and civil penalties and fines.
Transocean responded to the British firm's procedures deeming it "desperate" and "unconscionable". It added that Deepwater Horizon was a "world-class" platform, which was compromised by BP's very own "cost-saving decisions". The company announced it will also be taking legal action against BP and other firms involved, such as cement-maker Halliburton, to defend its role in the disaster. Cameron International took similar steps to defend its staff and equipment.
The oil spill triggered by the Deepwater Horizon platform's explosion last year caused an estimated 200m gallons (757m litres) of oil to spread in the waters of the Gulf of Mexico.