www.observer.guardian.co.uk Some of the world's most precious archaeological treasures - the ancient Egyptian tombs and temples at Luxor - are being devastated by salt water that is eating their foundations, scientists have discovered.

The temples of Amun, Luxor and Karnak, designated World Heritage Sites, have survived 4,000 years of arid desert heat but are now being destroyed by rising ground water.

The threat has been uncovered by American Egyptologists, who have warned that urgent action is now needed. Their view has been backed by Zahi Hawass, secretary-general of Egypt's Supreme Council of Antiquities. 'When I found out the Temple of Luxor and the Temple of Karnak were going to completely fall apart because of the rising water table, I was shocked,' Hawass said in an interview in Science.

The crisis has been caused by several factors, including climate change and the breakdown of the area's ageing sewer system. However, the most important threat has involved the recent, massive intensification of farming along the Nile and the widespread planting of sugar cane, a plant that flourishes in saturated soil. To boost harvests - up to three crops a year can now be grown - farmers have been inundating their fields all year round with water sucked from the Nile. Land that was once parched desert now sports massive, flourishing tracts of canes. Drainage canals are now never allowed to dry out, with the result that Luxor's water tables have risen several metres in the past few years.

Those waters now swirl around beneath the great temples of Karnak and Luxor and the Amun temple complex of Medinet Habu, as well as the tombs of the nearby Valley of the Kings. This is the second most important tourist destination in Egypt, after the Pyramids of Giza, and it is in dire peril.

The problem, say engineers, is that the ground water - which contains high levels of salt - is being absorbed by the temples' soft sandstone. When it evaporates, the salt crystallises, filling and then bursting the pores of the rock. After prolonged exposure, the rock disintegrates. 'You can see something like a tide mark produced by the water rising up the sides of the temples, getting higher and higher every year,' said Dr Nigel Strudwick, Assistant Keeper at the Ancient Egypt & Sudan Department at the British Museum. 'It is not just damaging the stone, it is destroying the decoration on the temples' walls faster than they can be studied. The obvious solution would be to raise up all the temples and put in damp courses, but that, of course, is not practical.'

In a bid to halt the worst of the water's impact, Swedish engineers - backed by أ‚آ£4m of US and European aid - recently began building a vast system of drainage ditches around the temples of Karnak and Luxor. With their breathtaking rows of giant columns and statues, these are the most important ancient sites on the east side of the Nile at Luxor. According to Christina Karlberg, of Sweco, the company involved, the work should be completed next year.

However, it is the threat to the Nile's western banks that really concerns Egyptologists. Its temples and tombs include Medinet Habu, the Amenhotep III mortuary temple, and the spectacular tombs of the Valley of the Kings and the Valley of the Queens. On this side of the Nile, the problem of sugar cane growing is most pervasive. Standing water can now be seen glittering in the ruins of the Amenhotep temple, for example, and scientists at Chicago University, say that the water is creeping toward the Valley of the Kings and the Valley of the Queens with disturbing prospects. It is here that the greatest - and the most vulnerable - sites are found, including the tomb of Tutankhamun.

These tombs, with their fabulous painting and engravings, are already being badly damaged by heat and moisture, brought in each year by thousands of tourists, say scientists. As a result, they have pressed the Egyptian authorities to ban sugar cane-growing in the area and urged that less water-intensive crops, such as beans, be sown instead. This idea has been rejected by the government, which heavily subsidises the country's sugar industry.

'It would be pretty much impossible to change agricultural practices,' admitted Hawass. 'We will have to focus on engineering solutions.'

The problem is that at present scientists are not sure what these solutions might be.