Lawyers for oil traders Trafigura finally abandoned attempts to keep secret a scientific report about toxic waste dumping in west Africa, that was shown to the Guardian.

Just after 7.30pm Carter-Ruck, libel lawyers for Trafigura, wrote a letter to the Guardian which said the newspaper should regard itself as "released forthwith" from any reporting restrictions. An MP revealed the report's existence to parliament this week, after the Guardian was hit with a "super-injunction" banning all mention of it and other UK media were then subsequently notified of, and therefore bound by it.

The Minton report, commissioned in 2006 from the London-based firm's scientific consultants, said that based on the "limited" information they had been given Trafigura's oil waste, dumped cheaply the month before in a city in Ivory Coast, was potentially toxic, and "capable of causing severe human health effects".

The study said early reports of large scale medical problems among the inhabitants of Abidjan, were consistent with a release of a cloud of potentially lethal hydrogen sulphide gas over the city. The effects could have included severe burns to the skin and lungs, eye damage, permanent ulceration, coma and death.

The author of this initial draft study, John Minton, of consultants Minton, Treharne & Davies, said dumping the waste would have been illegal in Europe and the proper method of disposal should have been a specialist chemical treatment called wet air oxidation.

Although the report was cautious, pointing out that unreliable press reports and "mass hysteria" might have led to exaggeration of alleged ill effects, its contents were unwelcome.

Trafigura subsequently did not use the report in the personal injury report in the claim against them and did not dislcose the report's existence.

It issued a series of public statements over the next three years saying the waste had been routinely disposed of and was harmless. Trafigura based this decision on other reports produced from an analysis of the slops obtained from the Probo Koala ship. Trafigura dismissed complaints of illness in a lawsuit brought by 30,000 inhabitants of Abidjan, before being forced last month to pay them £30m in compensation and legal costs in a confidential out of court settlement.

The oil firm then conceded in a public statement that the toxic fumes could have caused "flu-like symptoms" to the inhabitants. But it was accepted in an agreed statement by both sides that expert evidence did not back the more serious claims of deaths, miscarriages or serious injuries, made in previous official statements by the Ivory Coast and British governments and in a UN report.

Before the settlement announcement, Trafigura's lawyers Carter-Ruck obtained a super-injunction from a judge, banning the Guardian not only from revealing the existence of the Minton report, but also from telling anyone about the existence of the injunction.

They said the Minton report was confidential because it had been obtained for possible use in litigation. Trafigura said the report was only preliminary and had proved to be inaccurate. They said hydrogen sulphide in the waste could not have broken down into a dangerous gas after the dumping and that other experts had concluded: "no other chemicals were released in concentrations capable of causing significant harm to human health".

Carter-Ruck was unable to prevent the publication of internal company emails by the Guardian, which confirmed Trafigura executives had been aware in advance that their waste was hazardous, and knew that it ought to have received expensive specialist treatment. Company traders talked about making "serious dollars" from paying someone to take away their "shit".

Attempts by Carter-Ruck to suppress the Minton report led to a controversy about parliamentary privilege this week, when the law firm initially tried to prevent reporting of parliamentary questions tabled by the Labour MP Paul Farrelly. They later abandoned this attempt. Carter-Ruck was accused by MPs of potential contempt of parliament.

Tonight, Alan Rusbridger, the Guardian's editor, said: "I welcome the climbdown by Trafigura and Carter-Ruck. Now people can read the Minton report they will realise why it was in the public interest for it to be published. It has taken a five-week legal battle – involving journalists, lawyers, bloggers and parliament itself – to force this information into the open. Never again should a newspaper be threatened with contempt of court for reporting parliament. And judges should think again about the use of super-injunctions which are themselves secret. This is a good day for parliament, open justice and free reporting."

Pierre Lorinet, Trafigura's chief financial officer, told the Telegraph: "We decided that our best course of action at the time was to get the injunction, because we didn't want more inaccurate reporting on things which are very clearly wrong effectively. It is a heavy-handed approach, absolutely. With hindsight, could it have been done differently? Possibly. The injunction was never intended to gag parliament or attack free speech."