(AFP) - Cleaning up decades-long oil pollution and restoring environmental health in just one of Nigeria's crude-producing states will cost at least $12 billion, investigators said on Tuesday.

Bayelsa state, home to some two million people, "is in the grip of a human and environmental catastrophe of devastating proportions," they warned in a much-awaited report.

Lying in the Niger Delta region, Bayelsa is where oil was first discovered in Africa in the 1950s, and where companies Shell and Eni have operated for decades.

"Once home to one of the largest mangrove forests on the planet, rich in ecological diversity and value, the region is now one of the most polluted places on Earth," the report said.

"At least $12 billion" is needed to "clean up the soil and drinking water, reduce the health risk to people and restore mangrove forests essential to stopping floods."

The four-year investigation was carried out by the Bayelsa State Oil and Environmental Commission -- an international panel of experts and prominent figures who worked at the request of the local government.

It called on Shell and Eni, whose local subsidiaries still operate in the region, to pay a share of the bill.

"We are asking Shell's new CEO Wael Sawan, before selling off Shell's remaining onshore oil assets, to commit immediately to paying their share of the $12 billion bill," said the commission's chairman, John Sentamu, a member of Britain's House of Lords and former Archbishop of York.

In a written statement to AFP, Shell said it had not seen the report and could therefore not respond to its conclusions at this time.

Eni also said that it had not been consulted about the report and rejected allegations of "environmental racism" made by the commission.

In response to AFP's request for comment, Eni said it "conducts its activities according to the sector's international environmental best practices, without any distinction on a country basis."

Both companies have blamed most oil spills on sabotage and theft.

"Regardless of the cause of a spill, we clean up and remediate areas affected by spills originating from our facilities," a Shell spokesperson said.

Eni also said the company "undertakes to remedy in all cases" when spills occur.

- Litany of problems -

The report is based on over 2,500 pieces of evidence including 500 interviews and analysis of 1,600 blood samples from local people.

Over the years, "as much as one and a half barrels of oil has been spilled in Bayelsa for every man, woman and child living in the state today."

The wider Niger Delta, according to the report, has suffered the equivalent of the Exxon Valdez disaster every single year for 50 years.

The 1989 tanker disaster spewed nearly 11 million gallons (42 million litres) of crude oil off the coast of Alaska.

The report pointed to the potentially far-reaching impact on health from oil and gas pollution.

"Highly toxic contaminants that cause burns, lung problems and risk of cancer are widespread," it said.

One sample of groundwater contained toxic chemicals present at more than a million times safe limits.

- 'Systemic failings' -

Researchers blamed the crisis on "the systemic failings of international oil company operators with the complicity of Nigeria's political classes and a dysfunctional Nigerian regulatory state."

The amount paid by companies, the report said, should be based on the amount of oil pumped since commercial exploitation began and "perhaps weighed to reflect the company's pollution record."

"The enormous suffering caused by oil pollution in my kingdom pokes me, chokes me, and stares me in the face every day," said King Dakolo, a traditional ruler and chief in Bayelsa, in testimony to the commission.

"There is talk of paying for climate loss and damage amongst world leaders. Oil companies could start by accounting for the damage done in my state."

The report comes days after Britain's Supreme Court ruled it was too late for a group of Nigerians to sue Shell over a 2011 offshore oil spill.

The energy giant, which recorded its highest-ever annual profit this year, faces more legal battles in Britain, including against 50,000 Nigerian claimants suing over other spills.

© Agence France-Presse