To the untrained eye it looks like an ugly fungus, or worse, dog dirt.

But mushroom experts say the rare delicacy lurking under the soil of the South Downs is an untapped gold mine.

Better still, it could turn Sussex into the new Provence or Tuscany and give French and Italian traders a run for their money.

So sought after is the fungus in question that it has become known as the black diamond, with restaurants and connoisseurs willing to pay up to £150 a kilo for a taste.

Mushroom aficionados say Littlehampton, Worthing, Brighton and Lewes have become prize spots to cultivate and uncover this humble but luxurious truffle.

While the traditional truffle grounds of France and Italy are faltering under heatwaves and climate change, Britain is experiencing a good year for fungi.

However, with a bumper harvest ready to be reaped, unsuspecting landowners, farmers and foodies could be missing out on thousands of pounds.

Truffle expert Nigel Hadden-Paton said he recently uncovered a crop worth about £300,000 in a small wood in Wiltshire and he now has his sights set on the South Downs, which he believes could provide a fertile hunting ground.

He said: "It is an under-used, underground resource. Landowners could be sitting on a gold mine. I know that the French and Italians are wailing."

It seems there is a gap in the market.

Dino Pavledis, sous chef at Terre a Terre on East Street, Brighton, said he uses Italian truffles in the restaurant's risotto dish because there were no local suppliers.

He said: "We try to find as many local products as possible but nobody has come to us offering local truffles."

The South Downs are particularly well-suited to the fungi, which grows in chalky soil under beech, hazel, oak, birch and hornbeam trees.

The area has been prized for truffles since the 19th Century, when Patching Woods, near Clapham, was believed to be Britain's top spot for the exclusive ingredient.

A truffle archive, registered at Royal Botanical Gardens, Kew, also reveals finds in Brighton, Goring and Folkington, near Polegate.

Dr Paul Thomas, a truffle entrepreneur who runs biotechnology company Mycorrhizal Systems and who was featured making a successful bid on the Dragon's Den show on the BBC, said Sussex was ideal for truffle cultivation and he was looking to set up plantations in the county.

One hectare can earn the grower £12,000 a year, more than many other crops.

Dr Thomas said: "Truffles can make a lot of money. The truffles are out there but people just aren't looking. Sussex definitely has a suitable climate for truffle cultivation.

"We are very keen to locate farmers in Sussex and to work with them to establish partnership truffle plantations.

"They will have the benefit of all our cutting-edge technology and scientific expertise."

The British truffle is easier to find than the continental variety as it often breaks through the surface.

However, many amateurs often mistake a truffle for dog excrement or other unpleasantries, say the experts.

Amateurs will, nonetheless, need a truffle pig or trained dog if they expect to find significant amounts of this culinary delight.