Shares in BP jumped on Wednesday morning on news that an official report into the Deepwater Horizon disaster, due to be formally released at 5pm on Wednesday, will spread the blame for the oil spill.

An investigation jointly conducted by the Bureau of Ocean Energy Management, Regulation and Enforcement and the US Coast Guard concludes, in line with previous reports, that BP's contractors Transocean and Halliburton should share some of the blame for the tragedy, which killed 11 men and caused the worst offshore oil spill in US history.

Wednesday's Wall Street Journal said the report faults BP "for seeking to save time and cut costs without considering the ramifications". Transocean should not have continued its work on the well after encountering hazards, it adds.

"The lead cause of the incident was a failure of the cement at the bottom of the well to stop oil and gas from flowing up, the report finds. Halliburton was in charge of the cementing process, it adds.

The report is considered significant because the investigators are the regulators of offshore drilling.

BP's own report into the disaster, released last September, cited eight reasons why the Deepwater Horizon rig exploded – with much of the blame assigned to Transocean.

BP shares rose 4% today. Given its size and significance to the FTSE 100 Index, BP's rise added 12 points to the index alone, collectively up 80 points at lunchtime.

Analysts from Merrill Lynch suggested the report could reduce the prospect of a potentially ruinous gross negligence case against BP: "We see this report, if published as indicated by the Wall Street Journal, potentially changing the market perception on BP's ultimate cost on Macondo. Crucially, we believe that with a number of government reports now spreading the blame between BP and its contractors, the likelihood of a gross negligence case against BP is diminishing."