Global Witness said it had investigated the activities of Lichtenstein-based company Norsudtimber in the DRC, a former Belgian colony, for two years.
"Norsudtimber operates almost entirely illegally, in violation of the essential points of the Congolese Forest Code," the NGO claimed, adding that it used satellite images to trace activity in the forest.
Three Congolese firms controlled by Norsudtimber -- Sodefor, Forabola and La Forestiere du Lac -- rejected Global Witness's charge that they had carried out logging outside perimeters designated by satellite images.
"An image with roads outside the perimeter does not mean that there are operations outside the perimeter," they said.
"(...) Care also has to be taken to check whether roads which are visible on satellite images are recent or old."
While almost 80 per cent of the company's timber exports went to China and Vietnam between 2013 and 2017, Europe accounted for about 11 per cent, mainly Portugal and France, Global Witness said.
"It is not surprising that the law is flouted in DRC, but it is shocking that France does so little to put an end to its imports of illegally harvested timber," said a spokesman for the NGO, Jules Caron.
Global Witness also called on the French Development Agency (AFD), a financial institution run by the French state, to cancel a four-year, 18-million-dollar project that would violate "the goals of protecting the climate and forest."
The agency denied the accusation.
"This NGO has been attacking the AFD for several years over a programme designed with the Congolese authorities that aims to improve governance of the forest sector," AFD told AFP.
DRC is home to more than 60 per cent of the dense forests of the Congo basin, the world's second largest tropical rainforest after the Amazon.
Illegal logging is a major problem in many developing countries including Congo, where poverty and decades of instability, corruption and poor governance have put enormous pressure on natural resources.
Campaign group Greenpeace has warned that deforestation of the basin, one of the most carbon-rich regions in the world, threatens the fight against global warming.