Glencore, the multi-billion pound commodity giant, stands accused of profiting from child labour in a mine in the Congo, and paying the associates of paramilitary killers in Colombia, following a Panorama investigation.
Undercover filming showed children as young as ten working in the Glencore-owned Tilwezembe mining concession.
And sales documents show a Glencore subsidiary made payments to the suspected associates of paramilitaries in Colombia.
Glencore chief executive Ivan Glasenberg denied both claims.
He said the company had "never dealt, never paid, never met the paramilitaries in all our years in Colombia" and did "not profit from child labour in any part of the world."
Panorama secretly filmed miners climbing down mineshafts 150ft deep without any safety or breathing equipment to dig copper in the Tilwezembe concession.
Many of the miners were under 18. One boy said he was ten years old. International law prohibits anyone under 18 working in a mine.
Although Glencore says it stopped operating at the mine in 2008, because of the collapse in the price of copper, it still owns the concession.
Glencore chief executive Ivan Glasenberg said the mine had been taken over by local workers without its permission.
He said the child miners were part of a group of freelance miners who "raided our land in 2010… against all of our authorisation. We are pleading with the government to remove the artisanal miners from our concession."
He also denied that Glencore bought any of the copper from Tilwezembe.
Panorama tracked a lorry leaving the mine to a processing plant owned by one of Glencore's main partners in the Congo.
And documents obtained by the programme provide strong evidence that some of the copper from Tilwezembe was sent from the processing plant to a Glencore smelter in neighbouring Zambia.
At least 10 people were murdered when paramilitaries seized a patch of land called El Prado next door to Glencore's Calenturitas coal mining concession in 2002.
A Colombian court was told last year by former paramilitaries that they had stolen the land so they could sell it on to Glencore subsidiary Prodeco, to start an open-cast coal mine.
The court accepted their evidence and concluded that coal was the motive for the massacre.
Ivan Glasenberg refuted the court's ruling. He told Panorama: "The court is wrong. If that's what the court ruling was, I can assure you Glencore doesn't own the land. There is no coal underneath that land. Or if there is coal underneath that land then it is very deep and not mineable."
Mr Glasenberg also said his company had 'never intended to own the El Prado land'.
But during the investigation Panorama obtained sales contracts between Prodeco and the new occupiers of the land - whom the authorities believe were henchmen of the killers.
One week after that interview was filmed, Glencore admitted paying $1.8 million for 'improvements' to the new occupiers of the land in 2008.
The company also conceded that under the terms of a 2009 swap agreement it would have ended up owning El Prado, adding the deal was not completed because the Colombian authorities failed to keep their side of the bargain.
Glencore said it was asked to go ahead with the deal by the then President of Colombia, Alvaro Uribe, as part of a resettlement programme. The company said: "we have made clear we do not want this land and have no interest whatsoever in it for our business".
The company is now negotiating with the government for an exit from the El Prado deal.
Panorama: Billionaires Behaving Badly? is on BBC One, Monday 16 April at 20:30 BST. Watch online afterwards (UK only) at the above link.