Paintings of horses adorn the cave

The primitive art of the Lascaux cave complex in south western France survived untouched for nearly 18,000 years but became extremely vulnerable when the cave mouth was widened in the 1950s to accommodate up to a thousand daily visitors.

Two years ago, experts noticed a fungus spreading gradually along the floor, walls and part of the ceiling.

They tackled it with antifungal and antibiotic chemicals, but it was extremely resilient, and only now have scientists stopped it in its tracks.

However, after more than a year of decontamination the authorities say they have not managed to return the cave to its original state.

The origin of the fungus continues to baffle scientists, partly because such careful measures have been taken to preserve the cave complex.

It was closed to the public 40 years ago, after green algae started to grow on the walls - brought in by fresh air after the cave mouth was widened.

An air-conditioning system was put in place to maintain the environment, but despite this the fungus took hold.

Numerous fungicides and bactericides were used against it, but France's ministry of culture says it was "very hard to treat".

Agricultural pest

In a report in the French scientific journal La Recherche, scientists reveal that the fungus may have been introduced accidentally on the muddy boots of workmen overhauling the air-conditioning system.

It is thought to be a member of the Fusarium family, which is a well-known agricultural pest.

Two years after the fungus was first spotted, the culture ministry has confirmed that the historic monument with its frescos of bulls, horses and Palaeolithic animals is no longer under threat.

It says it has managed to contain the spread of the Fusarium, but still has to restore the cave's biological balance, which helped preserve the paintings in such remarkable condition for thousands of years.

It plans to bring together scientific experts in a range of fields to learn exactly how climatic, chemical, biological and other forces come together inside the cave.

It hopes this multi-discipline group will be able to get rid of the remaining fungus and prevent future contamination.