The owner of the oil rig that exploded in the Gulf of Mexico in 2010 claims BP hampered efforts to stop the resulting gush by misleading the US government about how many barrels of oil were flowing each day from the damaged well.
The Transocean corporation's assertions were filed in federal court in New Orleans, where a civil trial began last week to determine blame and how much BP, Transocean and others will pay for the April 2010 catastrophe that killed 11 workers and sent millions of gallons of oil spewing into the Gulf for 87 days.
"In short, beginning in late April and continuing throughout May 2010, BP repeatedly represented to source control decision-makers, Congress, the press and the public that 5,000 bpd was its best estimate of the flow rate," the Transocean filing said.
"It withheld numerous documents, analysis and estimates that would have allowed those outside BP to realise that BP's flow rate claims were misleading and fraudulent."
Transocean, which leased the Deepwater Horizon rig to BP, says the leak could have been stopped two months earlier. The motion filed on Friday seeks to limit or eliminate Transocean's liability for damages, and outlines a case for collecting damages from BP itself.
Transocean's filing says government officials attempted a method of stopping the flow that was destined to fail because oil was spewing at a greater rate than BP was publicly acknowledging. That method, known as "top kill", involved plugging the well by injecting drilling mud and solid material. The attempt failed.
Transocean, citing various documents and evidence including BP's recent guilty plea to criminal charges, said BP was well aware of estimates that much more oil was flowing, varying from 70,000 to 100,000 barrels a day.
Ultimately, a device known as a "capping stack" stopped the flow.
Transocean said but for BP's actions, the oil flow could have been stopped some time in May. BP has already pleaded guilty to manslaughter and other criminal charges and has racked up more than 24 billion dollars (£16bn) in spill-related expenses, including four billion (£2.6bn) in criminal penalties.
In the trial that began last Monday, Gulf Coast states and individuals and businesses hope to convince the judge that the company and its partners in the drilling project are liable for much more in civil damages under the Clean Water Act and other environmental regulations.