A United Nations investigation into toxic waste dumping in the Ivory Coast has challenged a key defence plank of the commodities trader Trafigura, which is facing the UK’s biggest class-action suit next month in London.

More than 20,000 Ivoriens are suing Trafigura in the High Court for damages after one of its ships, the Probo Koala, allegedly offloaded toxic “slops” in the west African port of Abidjan in August 2006 to a local company which dumped it in open-air sites.

A health crisis ensued in which, according to official estimates, 15 people died and more than 100,000 sought medical attention after exposure to the waste’s fumes.

The unedited report seen by the Financial Times – written by Okechukwu Ibeanu, the UN special rapporteur for toxic waste – said that the waste inside the Probo Koala was “hazardous” and “can be harmful to humans ... if serious exposure takes place”.

The report concludes that there is ”strong prima facie evidence that the reported deaths and adverse health consequences are related to the dumping of the waste from the Probo Koala”.

Trafigura said that in spite of offers of information to the rapporteur, the “flawed” report contains “premature, inaccurate and potentially damaging conclusions that are unsupported by verifiable evidence”.

The report, which looks into the effects of the Probo Koala’s waste on human rights, is due to be officially unveiled by the UN in Geneva on Thursday. Mr Ibeanu said it is not meant to ascribe liability.

The report said that Trafigura contracted the slops disposal to a local company, Tommy Ltd, which had been created “only shortly prior to the arrival of the Probo Koala” and possessed no previous waste disposal experience.

Tommy informed Trafigura it would discharge the waste in a dump where poor communities made a living out of recycling rubbish, says the report.

In 2007 Trafigura paid $198m to the Ivorien government as a contribution for damages suffered. No liability was accepted.

Trafigura, which has commissioned its own investigation, maintains that the dumped slops could never have caused the deaths and serious illnesses alleged.

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