We have received a complaint from solicitors acting for Trafigura about this article.

Read Trafigura's statement here

Documents have emerged which detail for the first time the potentially lethal nature of toxic waste dumped by British-based oil traders in one of west Africa's poorest countries.

More than 30,000 people from Ivory Coast claim they were affected by the ­poisonous cocktail and are currently bringing Britain's biggest-ever group lawsuit against the company, Trafigura.

The firm chartered the ship, Probo Koala, which transported the cargo to Ivory Coast in 2006.

An official Dutch analysis of samples of the waste carried by the Probo Koala indicate that it contained approximately 2 tonnes of hydrogen sulphide, a killer gas with a characteristic smell of rotten eggs.

The documents have been obtained by the BBC. One chemist told BBC Newsnight last night that if the same quantity and mixture of chemicals had been dumped in Trafalgar Square: "You would have people being sick for several miles around … millions of people."

Trafigura, which claims to be one of the world's biggest independent oil ­traders, originally issued statements in 2006 denying the tanker was carrying toxic waste. It said it merely contained routine "slops" – the dirty water from tank washing. Executives of the company lined up to specifically deny that the waste contained any hydrogen sulphide.

It was put by the Guardian to ­Trafigura's lawyers with 24 hours notice that the company had been making wrong statements about the toxic waste. The firm's ­lawyers, Carter-Ruck, then asked to be given more time, until 5.30pm tonight. When 5.30pm passed, they asked for another hour. By 7pm, they still remained silent.

They issued a statement at 7.38pm saying: "We have no intention of descending into a detailed debate as to the chemical composition of the 'slops'." They said such matters would be resolved at the high court trial and that Trafigura's position was "the slops did not and cannot have caused the deaths and widespread illnesses which have been alleged".

In the long-running and bitterly contested lawsuit, Trafigura has currently offered to pay anyone from Ivory Coast who can prove the toxic waste actually caused them to fall ill. The claimants' lawyer, Martyn Day, the senior partner at Leigh Day, a firm which specialised in group actions, says this concession will prevent the evidence detailing the true history of the Probo Koala from coming out in court. The case is due to be heard in October.

Court hearings were held in secret ­yesterday, after allegations were made that Trafigura had been attempting to "nobble" witnesses to induce them to change their stories. Trafigura in turn claims that its employees face reprisals and even threats to their lives.

Mr Justice McDuff said: "This case has attracted much media attention and aroused strong feelings for many. ­However, I have acceded to an application to hold part of the hearing in private because on the evidence I have heard certain parties to the litigation have felt intimidated or threatened with reprisals."

He said the secret hearing involved "applications for witness anonymity and other witness protection".

A previous hearing had issued an injunction banning Trafigura from contacting any of the claimants. This followed the submission of evidence that one of the claimants had been flown business class from Ivory Coast to Morocco, put up at a luxury hotel, and offered money. The claimant said he had also been interviewed in the Morocco hotel by Simon Nurney, partner at ­Macfarlanes, Trafigura's solicitors.

Macfarlanes denied last night there had been any impropriety.

They said: "Macfarlanes has valid and exceptional legal reasons for agreeing to meet the individual referred to by the Guardian. There was (and is) evidence that fraudulent and exaggerated claims were being made against our client. We therefore had the right, and indeed duty, to investigate by interviewing the claimants.

"Macfarlanes arranged to meet the individual in Morocco. Given the extreme sensitivity of any meeting between that individual and Macfarlanes, he specifically requested that the meeting should take place outside the Ivory Coast. ­Macfarlanes were represented in Morocco by one of our partners, who acted with the full knowledge and support of the firm and the ­client. This partner and another member of the legal team were present at the meetings in Morocco. Macfarlanes paid the individual's travel and related expenses in relation to these meetings, but neither made nor offered any other payment or inducement."