The geologists have measured movement on the steep back slope of the fortress.

They have found that the land is sliding down at a rate of about one centimetre (0.4 inches) a month. Scientists say this is quite fast and is a precursor to a major landslide.

But they say they cannot predict when this might happen and will try to determine that during the next stage of their research.

Claim denied

However, a Peruvian preservation official has firmly denied that landslides pose an immediate threat.

The Director of the National Institute of Culture in Cuzco, Javier Lambarri, said the ruins are monitored for seismic activity 24 hours a day. "As of now, we have no report that there is an imminent danger," he said.

However, Mr Lambarri did admit that there was a geological fault line in the region that could cause difficulties.

"We know that there is a fault line that crosses the entire city and that could cause a problem," he said.

Preservation debate

Already, a debate exists over how best to preserve Machu Picchu for future generations.

Last year, the authorities decided to limit the number of visitors to the site because of fears of erosion. But the measure has not satisfied conservationists.

They have warned that more needs to be done and they have campaigned against plans to build a cable car up to the fortress.

One Peruvian presidential candidate has promised that if he wins next month's election, he will stage his inauguration at Machu Picchu.

Now, perhaps, his plans may have to be revised.