Millions of gallons of oil spilled into the Gulf following the April 20 explosion on the drilling platform [Gallo/Getty]

BP accepted some responsibility but generally assigned blame to others for the Gulf of Mexico oil disaster in an internal inquiry released on Wednesday.

The oil giant's investigation admitted some mistakes that it said led to the April 20 explosion, which killed 11 workers and caused the worst spill in US history, but it came down hard on contractors working for BP, including Transocean.

The report acknowledged that BP workers on the Deepwater Horizon oil rig misread pressure data which indicated that a blowout was imminent.

But it also highlighted the responsibilities of other companies involved, including Transocean, which leased the rig off the Louisiana coast, and Halliburton, which cemented the well.

The three firms previously blamed each other for the disaster at a hearing in the US Congress in May.

Legal implications

Wednesday's findings of the investigation by Mark Bly, the firm's head of safety and operations, is viewed as key to how BP defends itself against legal proceedings involving the spill.

Peter Hutton, an analyst on the oil industry for NCB Stockbrokers, said he expected that BP would admit some failures.

"But it will be crucial for BP to differentiate errors from a systemic process or culture which allows the charge of 'gross negligence' which has implications for the level of punitive damages," he said.

Manouchehr Takin, a specialist in petroleum production with the Centre for Global Energy Studies, said BP's internal report was "impartial as a technical evaluation".

"One cannot blame one or two, and one cannot say there is no one to blame," he said.

The leaking Macondo well has now been secured but the disaster is being examined in a string of court cases and probes, including a criminal investigation being carried out by the US Department of Justice.

BP has already spent $8bn trying to contain the disaster and has forecast that it will eventually cost the group more than $32.2bn after clean-up costs and compensation are taken into account.

US politicians have accused the oil giant of sacrificing safety to improve its profit margin - a charge denied by Tony Hayward, BP's chief executive, during a session in congress in June. 

The US government is currently conducting its own investigation into the oil disaster.