The report from the US Coast Guard and offshore oil regulator says BP was solely to blame for 21 of the 35 contributing causes behind the explosion in the Deepwater Horizon rig in the Gulf of Mexico, and was partially to blame for another eight.
It states that a number of decisions BP took to cement its deepwater Macondo well complicated the operation and may have led to the ultimate well blow-out that led to the vast leak.
"BP's cost or time saving decisions without considering contingencies and mitigation were contributing causes of the Macondo blowout," the report said.
But BP shared the blame with US contractor Halliburton, which was responsible for the cementing, as the report slammed its work on the well.
Last year's explosion on the BP-leased Deepwater Horizon rig killed 11 workers and spewed more than 4m barrels of oil into the sea.
The report highlighted a litany of errors that preceded that, concluding that BP failed to communicate decisions regarding the cementing that increased operational risks to Transocean, the contractor that owned and operated the rig.
Transocean workers missed an opportunity to address the cement problems when they misinterpreted a critical test of the well's cement barriers.
But the report acknowledged that the Macondo project was plagued by problems even prior to the spill.
It ran significantly over budget and had shown signs that all was not right. The report disclosed an email from a BP engineer that labeled Macondo as a "a nightmare well which has everyone all over the place."
BP worked with Halliburton to design the cement job.
Because the well was over budget, "BP sought to minimize these losses by reducing the volume of cement it pumped into the well" and a key analysis recommended by a Halliburton engineer was skipped, according to the report.