It has survived the collapse of the sophisticated civilisation that built it, centuries of consumption by the suffocating jungle and the nihilism of the Khmer Rouge, who beheaded its stone Buddhas and used its walls for target practice. Now, Cambodia's awe-inspiring Angkor Wat complex is facing the biggest threat in a millennium - the fastest-growing tourist onslaught of any World Heritage site, which conservationists warn is already damaging its treasures irreparably.

In 1993, after Angkor was added to Unesco's World Heritage List, just 7,650 intrepid visitors ventured to the site. Last year Sokimex, the oil company controversially granted the entrance concession on behalf of the government's Apsara Angkor management, sold almost 900,000 tickets worth $25m (£12.8m), with British travellers making up the fourth biggest contingent behind South Koreans, Japanese and North Americans. Three million visitors are expected in 2010.

...Kerya Chau Sun, director of tourism at Angkor, said: 'We are finalising regulations for controlling visitors. We will train guards to watch the temples and educate visitors to help us protect the monuments.'

However, John Stubbs, who has spent 15 years working at Angkor with the New York-based World Monuments Fund, said: 'Tourism is already out of control, and unless the Cambodian government takes some pretty radical action to rein it in now much of Angkor's magic and heritage could be lost forever.'