The fungus is believed to have been introduced after contractors began to install a new air conditioning system that was meant to preserve the precious 17,000-year-old cave paintings from the heat and humidity generated by their many visitors.
The historical importance of Lascaux is immeasurable and any damage to its art would have serious repercussions given the cave's status as an evolutionary icon for the development of human art and consciousness.
The figures are so modernist in design that when Picasso emerged from the cave soon after it was first discovered in 1940 he exclaimed: "We have invented nothing."
Some experts who have seen the damage claimed that the French authorities had deliberately played down the scale of the problem because of their embarrassment at allowing it to happen to a World Heritage Site.
At one time, the fungus covered the floors of the entire cave system near Perigueux in the Dordogne in central France, although the curator of Lascaux insisted yesterday that the infestation had now been brought under control.
"The fungus appeared very suddenly. All the floor was covered as if in snow, but only the floor, not the paintings on the walls," said Dr Jean-Michel Geneste, director of the French government's National Centre for Prehistory. "We think that now there is no risk to the paintings. A few years ago we thought there would be a risk to them because of this fungus."
However, other visitors to the caves are not convinced that the fight against the fungus, which first appeared in 2001 just months after a new air-conditioning system was installed, has been won.
"They tell us the cave's condition is stable. But that's what they say about Ariel Sharon," said one anonymous expert quoted in a special report by Time magazine.
The magazine also claims that French officials last month admitted for the first time that the fungus had spread from the floor to the wall paintings.
One photograph published by Time shows the fungus apparently attacking a prehistoric horse painted on one of the walls of the cave's main gallery.
Teams of scientists are working in shifts to carefully remove visible filaments of the fungus - a species identified as Fusarium solani - by meticulously plucking them from the wall of the cave by hand, the magazine says.
"One knowledgeable visitor to the cave last month not only saw Fusarium on the paintings, but noticed a greyish tinge to formerly black surfaces where growths had been removed," the magazine says.
The archaeologist Paul Bahn, an expert on cave art, said: "This is extremely worrying. If the fungus is reaching the paintings, it's potentially catastrophic."
But Dr Geneste denied that there had been any damage to the painted figures of prehistoric bulls, horses and reindeer which are depicted running across the cave's walls and ceilings.
"The paintings are really fresh. There is no damage to the paintings, although there was a danger if the fungus was allowed to develop over many years," Dr Geneste said yesterday.
The fungus first appeared in 2001 and its sudden growth coincided with work to install a new active method of conditioning the internal atmosphere of the cave using fans to draw air through the underground cavern.
To accommodate the machinery, the contractors removed a roof over the entrance but a torrential downpour caused rainwater and mud to be washed into the cave, possibly introducing fungal spores in the process.
"The construction site was run like someone redoing a bathroom. The entrance to the cave was like a swamp and there was construction waste all over the place. It was like an apocalyptic vision," Rosalie Godin, a local art restorer, told Time.
Eventually the fans were taken out and the cave's curators were faced with the difficult job of trying to fight the fungus with antibiotic chemicals applied to the walls and quicklime spread on the floor, neither of which proved a success.
In the end Dr Geneste said that the best method turned out to be the mechanical removal of fungal filaments by hand, with the help of a special vacuum cleaner.
The device directs a high-pressure spray at the fungus which is then immediately sucked into sealed bottles that are removed from the cave.
After the cave was discovered, many thousands of visitors came to see its paintings each day but the increase in temperature and humidity took its toll, leading to the cave's first closure in 1962.
In 1983 a facsimile cave, known as "Lascaux 2", opened nearby to accommodate the public. Meanwhile, scientists and scholars studying the original cave were limited to five a day, five times a week.
However, the complete closure of Lascaux to all outsiders has led some cave experts to criticise the apparent secrecy over the type of conservation work being carried out inside the cavern.
Dr Geneste said that he had asked for an independent report on the conservation work to be published to dispel accusations of a cover-up. "I'm asking for the authorities to put it on the internet, even though it was meant to be confidential," he said.
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