BP has blamed the catastrophic Gulf of Mexico oil spill on a chain of events rather than a single action as it reiterated its previous comments that all companies involved must shoulder responsibility.

In a report into the causes of the Macondo well tragedy, BP identified a 'series of complex events' rather than one mistake, which led to the explosion and death of 11 workers in April.

The report - based on a four-month investigation led by Mark Bly, BP's head of safety and operations - found that the results of the negative pressure test prior to the explosion were incorrectly read by BP and Transocean, while a 'bad cement job' and a failure of the casing at the bottom of the well allowed oil from the reservoir into the production casing.

It was the job of the blow-out preventer to automatically seal the well in the immediate aftermath of the explosion, but it failed to operate 'probably because critical components were not working', the oil giant added.

Crucially, BP said the design of the well was unlikely to have contributed to the disaster.

Outgoing chief executive Tony Hayward, who came under fire for his handling of the spill and subsequently resigned, said the investigation provides 'critical new information' to help understand the causes of the accident.

'It is evident that a series of complex events, rather than a single mistake or failure, led to the tragedy. Multiple parties, including BP, Halliburton and Transocean, were involved,' he said.

BP hired the Deepwater Horizon drilling rig from Swiss-based Transocean, while Halliburton was subcontracted to cement the wellhead on the seabed.

The blame battle raged on as BP pulled up Transocean's rig crew for failing to recognise and act on the influx of hydrocarbons into the well over a 40-minute period, until they were already rapidly flowing to the surface.

Incoming chief executive Bob Dudley said: 'We have said from the beginning that the explosion was a shared responsibility among many entities. This report makes that conclusion even clearer.'

BP has now vowed to step its safety operations and said it is willing to invest whatever it takes to ensure it achieves its goal.

So far, the investigation team has proposed a total of 25 recommendations designed to prevent a recurrence of such an accident, including the strengthening assurance on blow-out preventers, well control, pressure testing, emergency systems, cement testing and personal competence.

'It will be incumbent on everyone at BP to embrace and implement the changes necessary to ensure that a tragedy like this can never happen again,' Dudley added.

BP can take some comfort in the news that ratings agency Fitch has upgraded its credit rating to A from BBB with a stable outlook, after confidence in the company grew following a permanent plug on the well.

Fitch also cited 'the substantial progress that BP has made to date in building up liquidity to address potential financial payments', including its claims fund and asset-disposal programme.

To date, the clean-up costs in the Gulf of Mexico have totalled $8 billion.

BP shares were up more than 2 per cent in the wake of the findings.