The Holland-based firm said independent tests showed the chemical sludge met international safety standards.

The environmental group Greenpeace maintained pressure over the scandal by blockading the Probo Koala, the tanker that carried the waste, in an Estonian port to demand an EU investigation.

Ten people, including two French Trafigura executives, have been charged with violating toxic waste laws and have been imprisoned in Abidjan.

Thousands suffered from vomiting, stomach pains and breathing difficulties after the black sludge was dumped around the town of Abidjan.

The Ivorian government resigned following a wave of public anger at the dumping.

Inadequate controls

Independent experts say the sludge contained hydrogen sulphide, a poisonous chemical that smells like rotten eggs.

But Trafigura said tests conducted by an independent Dutch laboratory run by Saybolt and an unnamed French company showed hydrogen sulphide was not present, disproving what it called "speculation in the media and the activist community".

The firm said in a statement: "Trafigura does not accept that it has acted improperly in any way. It maintains that the composition of the 'chemical slops'; gasoline, spent caustic soda and water, is a normal by-product from the cleaning of gasoline blendstock cargo.

"The slops are entirely in line with industry practice and international regulations. ... Trafigura's own analysis, and that of a French company, show that Hydrogen Sulphide level was not detected in the 'chemical slops'."

Greenpeace said the Probo Koala should not have been allowed to sail away from Ivory Coast and that international controls on toxic waste were inadequate.

Jacob Hartman, a Greenpeace campaigner, said: "The ship must be investigated, checked for further toxic cargo and detained until the full chain of responsibility for this deadly disaster is revealed and the guilty are prosecuted."