The llama-like creatures act as bouncers on his farm near Tetbury, in Gloucestershire, bonding with the sheep, ever ready to protect them against predators such as foxes, which used to kill as many as 50 newborn lambs every year.

Unlike sheep, alpacas happily confront foxes, chase them and try to trample or kick them. And while the prince’s staff have not yet found any battered fox bodies in the hedgerows, they haven’t lost any lambs either since the alpacas arrived.

Alpacas, which are related to camels, are domestic animals first bred from wild vicuna in South America. They are edible when young but are reared mainly for their fine wool. They look like a cross between a small llama and a large, long-necked sheep and can grow to 6ft tall.

Duchy Home Farm has been run organically since 1986 and is renowned as one of the most environmentally friendly farms in the country.

Foxes, though, are a serious problem. Until 2005, when the ban came into force, the Beaufort hunt helped to control the fox population in rural areas of Gloucestershire.

The new laws did not, in fact, outlaw the extermination of foxes by other methods such as shooting. They may also be hunted with other species. The Warwickshire hunt even experimented with giant eagle owls.

Charles’s alpacas do not hunt. Instead, they are used as sentries, standing guard over his collection of sheep, which includes some valuable rare breeds. If one spots a fox, they all charge over to administer a group kicking.

All four animals are male geldings, worth about £500 each. Breeding animals are too valuable for such work, with pregnant females fetching £3,500 or more and stud males worth up to £30,000 depending on pedigree.

Rob Bettinson, former president of the British Alpaca Society and now owner of Toft Alpacas, keeps a breeding herd of 150 on his farm in Dunchurch, Warwickshire. “Alpacas were introduced to Britain only about 12 years ago but the national herd is now 17,000 strong,” Bettinson said. “They are popular as pets and for breeding and farmers increasingly use them as sentries to guard other animals.

“If a fox enters the field they will all start walking or running towards it. Alpaca can kick and trample very hard so the fox usually gets the message pretty quickly and clears out.”

Such protective behaviour seems instinctive to alpacas, which appear to bond with any animals grazing with them in the field. Farmers have reported watching alpacas rounding up sheep and chickens and then standing guard over them when danger looms.

They may have their work cut out on the prince’s farm. There are an estimated 240,000 adult foxes in Britain and 425,000 cubs are born each year.

Because the adult population is fairly stable, this means that about 425,000 foxes die each year – mostly through disease, starvation and road accidents.

In rural areas gamekeepers kill about 75,000 a year. Hunts, when legal, killed about 16,000.

Richard Kempsey, production manager at Clarence Court, a company producing speciality eggs from birds including ostriches, geese, ducks and rare breeds of hens, suffered years of predation at his farm near Truro, in Cornwall, until he bought two alpacas.

He said: “We have 2,000 birds on site and the foxes are desperate to get at them but the alpaca keep them away. We have named them William and Harry.”

The Royal Society for the Prevention of Cruelty to Animals was nonplussed when asked whether using alpacas was cruel to foxes. “Using an imported South American camel to chase British foxes is so bizarre, we have not even thought about it,” said John Rolls, the society’s director of communications.

A fox’s worst nightmare

- Alpacas are fluffy and sociable but can be fierce. They chase away intruders and some spit globules of semi-digested vegetation mixed with stomach acid

- They live in families consisting of a single male, several females and young, known as cria

- They originate from the Peruvian Andes, where they thrive at altitudes of 10,000-15,000ft