A corner of the granite intihuatana, or "hitching post of the sun", was smashed when a camera platform collapsed on top of it.

The irregular carved pillar, one of several erected by the Incas at major sites, holds special significance as it was the only intihuatana not to have been discovered and destroyed by the Spanish conquistadores.

The structure is thought to have been an astronomical device, possibly used to predict the solstices of the sun, and is reputed to imbue with energy those who approach it.

More than a dozen fragments of rock will be painstakingly reattached, but the changeable and intemperate climate means they could eventually fall off again.

"The damage can probably be repaired but [the intihuatana] will never be quite the same again," said archaeologist Luis Barreda Murillo.

The accident happened during filming of a commercial for Cusqueña beer, which is produced in the Cusco region where Machu Picchu is situated.

Exhausted trekkers, arriving at the ruins after a four-day mountain hike along the Inca trail, watched thirstily as a chilled bottle of the lager and a glass were placed on the intihuatana.

Half a dozen members of the crew held up boards to reflect the bright sunshine, and the hapless cameraman was hoisted into the air to film from above.

He was slightly injured when the overloaded gantry collapsed on the rock.

The commercial was intended as part of a series showcasing the beauty of the Cusco region's historical legacy.

Carlos de la Flor, the managing director of the Cervesur company that makes the beer, says his firm was not to blame for the accident but will do whatever it can to help repair the damaged stone.

The Walter Thompson agency running the advertising campaign has not commented publicly on the incident.

Historians immediately called for a ban on commercial filming at Machu Picchu, South America's most spectacular archaeological site.

The area has been designated a Unesco world heritage site and tourists are strictly policed: backpacks and food are forbidden, and whistle-blowing guards prevent visitors from walking on the walls.

However, there are currently no limits on the commercial exploitation of the ruins, perched dramatically between two Andean peaks above the Urubamba river.

Unesco, alarmed by the rapidly expanding tourist infrastructure at Machu Picchu, has threatened before to designate the area an endangered site.

An unsightly hostel and latrines have been built on the Inca trail, and a planned cable car will increase the tourist crowds still further.

The latest incident will have done little to convince the organisation that the Peruvian authorities take their stated commitment to protect the park seriously.

Built in the 15th century by the Inca emperor Pachacuti, Machu Picchu was abandoned before the 1532 arrival in Peru of the Spanish forces led by Francisco Pizarro.

The forgotten settlement therefore avoided the ravages of the conquistadores and was found to be exceptionally well preserved when it was excavated after being rediscovered in 1911 by the US explorer Hiram Bingham.