Lacking standards to weigh costs against safety, BP and its partners made critical errors leading to the largest offshore oil spill in US history, according to a scientific panel report obtained by Reuters on Tuesday.

Interim findings from the National Academy of Engineering and the National Research Council attribute the massive spill to workers' decisions to move ahead with temporary abandonment of BP's doomed Macondo well despite warning signs from a key test of well integrity.

'The various failures mentioned in this report indicate the lack of a suitable approach for anticipating and managing the inherent risks, uncertainties ... associated with deepwater drilling,' the report said.

Government regulators also failed to provide adequate oversight for deepwater drilling, according to the report.

Ultimately, the report said there were not enough checks on decisions involving abandoning the well and there was no standard to weigh the trade offs between costs and safety for the project.

'Many of the pivotal choices made for the drilling operation and temporary abandonment of the well were likely to result in less cost and less time relative to other options,' the report said.

An April 20 explosion on the Deepwater Horizon rig killed 11 workers and ruptured the undersea Macondo well, unleashing millions of barrels of oil into the Gulf of Mexico over the summer.

A BP worker was responsible for overseeing costs and schedules for the well, as well as issues relating to safety and well integrity, the report said.

Despite testimony that 'safety was never compromised,' the report said that drilling risks were not fully recognised. The panel said it will consider whether a separate system of oversight is necessary to ensure concerns about cost do not compromise safety.

A BP spokesman said in a statement the company 'will reserve further comment until the committee's interim report has been released publicly and we have had the opportunity to review it.'

The report is scheduled to be officially released on Wednesday. It follows findings released by the White House oil spill commission last week that said workers at the companies did not cut corners on safety to save money.

The scientific panel's report is critical of BP's choice to use a long-string well design in a deep high-pressure well. It also questions the decision not to run a bond log to assess the stability of cement in the well.

The companies involved with drilling the Macondo well have all traded blame over responsibility for the rig accident.

Transcocean, the owner of the Deepwater Horizon rig, and Halliburton, which did cementing on Macondo, have both charged that BP's well design was a major factor in the explosion.

BP has denied these claims, instead faulting Halliburton for a flawed cementing job on the well. BP and Transocean also disagree on who was in charge of interpreting the pivotal negative pressure test, which indicated severe problems with the well.

But the report does not explicitly assign company fault for various mistakes that occurred in the run-up to the accident. Transocean and Halliburton did not immediately respond with comment on the report.

Regarding the cementing, the report cited several possible reasons it may have failed, including the type of cement used and the decision to use only six centralisers.

In addition, the multiple government agencies overseeing drilling operations were unclear about their specific duties creating gaps in oversight, the report said.

Interior Department inspectors also were not sufficiently trained to enforce deepwater drilling regulations, it said.

The Interior Department commissioned this report from organisations in the National Academy of Sciences, which advises the federal government on medical and scientific policy. The panel's final report is due in June 2011.-Reuters