Eleven people were killed and 17 people were injured in the explosion
A "sequence of failures involving a number of different parties" was to blame for the Deepwater Horizon oil spill in the Gulf of Mexico, BP says.
An investigation carried out by BP said it was responsible in part for the disaster, but it also blamed other companies working on the well.
BP faces billions of dollars-worth of legal claims for compensation over the spill, the worst in recent US history.
An estimated 4.9m barrels of oil leaked into the Gulf after the blast.
The well was capped on 15 July, and an operation to permanently seal it is due to take place in the next few weeks.
In the 193-page internal report released on its website, BP said that decisions made by "multiple companies and work teams" contributed to the accident, which it said arose from "a complex and interlinked series of mechanical failures, human judgements, engineering design, operational implementation and team interfaces".
BP was leasing the Deepwater Horizon rig from Transocean, and its cement contractor was Halliburton.'Bad cement'
The report, conducted by BP's head of safety, Mark Bly, highlighted eight key failures that, in combination, led to the explosion.
BP said that both BP and Transocean staff incorrectly interpreted a safety test which should have flagged up risks of a blowout.
"Over a 40-minute period, the Transocean rig crew failed to recognise and act on the influx of hydrocarbons into the well" which eventually caused the explosion.
BP criticised the cementing of the well - carried out by Halliburton - and repeated previous criticism of the blowout preventer.
"To put it simply, there was a bad cement job and a failure of the shoe track barrier at the bottom of the well, which let hydrocarbons from the reservoir into the production casing," said outgoing chief executive Tony Hayward.
"The negative pressure test was accepted when it should not have been, there were failures in well control procedures and in the blowout preventer; and the rig's fire and gas system did not prevent ignition," he said.
The blowout preventer that failed was recovered from the Gulf of Mexico on Saturday and transported to a Nasa facility near New Orleans where it will be placed in the custody of the US Justice Department and examined.
Transocean and Halliburton have not yet commented on the report.'Lessons for future'
BP's incoming chief executive Bob Dudley said the report proved that the explosion was "a shared responsibility among many entities".
The company said it had accepted all the recommendations in the report, and would implement them worldwide.
"We are determined to learn the lessons for the future and we will be undertaking a broad-scale review to further improve the safety of our operations... to ensure that a tragedy like this can never happen again," Mr Dudley said in a statement.
BP's shares rose after the report was published at 1100 GMT (1200 BST).
BP says dealing with the aftermath of the spill has cost $8bn (£5.2bn), and it has already paid out about $399m in claims to people affected by the spill.
The leak caused widespread disruption to jobs in the fishing and tourism industries along the US Gulf Coast.
A national commission is expected to submit a report to President Barack Obama by mid-January next year. A Congressional joint investigation will submit a report later than month.
The US Justice Department is also investigating the disaster, but its investigations will only conclude when lawyers and investigators have found evidence, or not, of criminal wrongdoing.