Centuries of grazing has turned the South Downs into its famous billiard table green, a grass carpet that rolls across the Sussex hills to the sea.

But as farmers started turning their backs on sheep, which once numbered tens of thousands, the delicate chalk grassland began its losing battle with trees and scrub.

Today's downland is a magnet for invading hawthorn, ash, sycamore, blackthorn and plants that have self-seeded from suburban gardens.

Now conservationists hope to reverse the trend by creating a 750-strong "flying flock" to combat invasive trees and scrub, and restore grassland at wildlife sites and beauty spots.

Groups of sheep from the Sussex Wildlife Trust's flock will be sent to trouble spots, first to nibble away young trees and other plants and later to maintain the turf.

Trust shepherd Gary Baldock said the flock would comprise "restoration grazers" from hardy upland breeds and "maintenance grazers" from traditional downland varieties.

Restoration grazers will include threatened Herdwicks, from the Lake District, a breed devastated by the culling policy during the foot-and-mouth crisis.

The initial main maintenance grazer will be the Wiltshire, a now rare, old English downland variety but one that has staged a comeback in recent years.

Sussex's own Southdown, another maintenance grazer, is in short supply but should eventually form a large part of the flock.

The rare Southdown, first bred by John Ellman at Glynde, near Lewes, in the 18th Century, was once the mainstay of downland farming.

Mr Baldock said: "The idea is that when we get the reserves up to how they should be we will be able to replace the Herdwicks with Southdown and other maintenance grazers."

At first the flock, based at Malling Down, near Lewes, will graze the trust's own nature reserves but if the project is a success the sheep could be used at other threatened sites.

The trust has launched a campaign to raise £70,000 to start the flying flock and, possibly, add six Sussex cattle.

As well as preserving flower-rich downland and helping rare breeds survive, the longer term ambition is to market South Downs-reared meat to Sussex shoppers.

About 90 per cent of chalk grassland, one of the most biodiverse habitats in Britain, has been lost since the Second World War, partly due to the decline in livestock farming.

One square metre of unspoiled downland can contain as many as 40 different plants, among them rarities such as the Fragrant Orchid and Round Headed Rampion.

Trust reserves officer Steve Tillman said grazing was crucial and the most sustainable way to preserve grassland, plants and animals.

He said: "The South Downs were built on grazing. If you were around 100 years ago there would have been thousands and thousands of sheep and there would not have been any scrub. The Downs would have been like a bowling green.

"If we can get 100 or 200 sheep grazing on a hillside, they will be nibbling on the hawthorn and look nice as well."