Unesco, the world cultural body, has threatened to humiliate France by placing the Lascaux caves – known as the "Sistine Chapel of prehistory" – on its list of endangered sites of universal importance.

The Unesco world heritage committee, meeting this week in Quebec, has given the French government six months to report on the success of its efforts to save the Lascaux cave paintings in Dordogne from an ugly, and potentially destructive, invasion of grey and black fungi.

At the same time, a scientific committee appointed by the French government has conceded that an elaborate treatment with a new fungicide in January failed to stop the mould advancing through one part of the caves.