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.