Rubber manufacturer Firestone has been accused of buying rubber from plantations illegally occupied by ex-combatants from Liberia's civil war.
It is one of three companies that the Liberian government and the United Nations have accused of profiting from the illegal rubber-tapping.
Their report says human rights are being violated at plantations across the West African country.
On two of the sites, workers are treated like slaves, the UN said.
Rubber is Liberia's biggest export and Firestone is by far the biggest producer.
But after more than a decade of civil war, which ended in 2003, the rubber industry is in a terrible state.
The situation is worst at two plantations occupied by former combatants beyond the reach of the law.
A UN official described the state of affairs at the Guthrie and Sinoe plantations as "disastrous", saying tappers worked in "conditions of slavery".
Firestone, owned by the Bridgestone Corporation, does not run these plantations, but it is accused of buying rubber from them.
Liberian Agriculture Minister Christopher Toe said the government had been reassured that Firestone had not bought materials from the sites, but that this was now in doubt.
"Regrettably, the evidence that we have gathered, especially in the last four to six weeks, is contrary to that assertion that they were not involved," he said
"There will be a directive to them [Firestone] that they are not going to be engaged in this type of activity."
However, the manager of Firestone in Liberia, Patrick Rodrigo, rejected the allegations.
He said Firestone did not buy any rubber from the Guthrie plantation, but he did not know if the company bought from the Sinoe site.
This is perhaps the nub of the problem. There is no system of checking the origin of rubber in Liberia.
The existence of former combatants illegally tapping rubber is a major blot on Liberia's path to normalisation.
And while they can still find a market, they are a problem that will not go away.
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