IT will take 40 years for dolphins in the Gulf of Mexico to recover from the devastating Deepwater Horizon disaster, an international study has found.

As much as 134 million gallons of oil spilled out from under the rig in 2010 during the final phases of drilling an exploratory well off the US coast, with escaping methane gas sparking a major blaze.

Eleven people died and thousands of marine mammals were also killed, with BP forced to pay out more than $60 billion in liabilities.

Now a study coordinated by the US National Oceanic and Atmospheric Administration (NOAA) has revealed the "unprecedented" impact on local wildlife.

The research, involving experts from St Andrews University, found the dolphin population in the Barataria Bay area of the Gulf of Mexico will have reduced by 50 per cent within the decade following the spill, with full population recovery expected to take 40 years.

Meanwhile, one quarter of the current population are underweight and 17 per cent are in "poor or grave" condition.

Professor Ailsa Hall, director of the Sea Mammal Research Unit at St Andrews, was an expert advisor on the Deepwater Horizon Natural Resource Damage Assessment (NRDA) and took part in the latest study.

She said: "My assistance was required to provide advice in relation to how assessing the damage to the bottlenose dolphins and large whales that inhabit the Gulf of Mexico and that were exposed to the oil should be tackled.

"My research expertise as a marine mammal epidemiologist and toxicologist was sought to provide independent critical review of the proposed work. I was therefore able to provide analytical input into the scientific approach taken by the NOAA scientists, to overview their research plans and to assist in interpreting their findings.

"The challenges faced by the NOAA scientists in determining whether the oil had caused significant effects on the health and survival of the dolphins and whales in the Gulf of Mexico was immense."

Dr Len Thomas, of the Fife university's Centre for Research into Ecological and Environmental Modelling (CREEM), added: "CREEM worked as part of a large team to predict the long-term damage to marine mammal populations from the oil spill.

"Our first challenge was to integrate multiple sources of information from the relatively well-studied dolphin populations around the Mississippi delta to assess the current population health and predict how this might change in the future.

"The second challenge was how to deal with the many other dolphin populations, and other species in the Gulf, about which much less is known.

"Despite all the uncertainties, it is clear that many populations of marine mammal were badly affected by the oil spill, and that these negative effects will persist for many years into the future."