The chalk grasslands, which stretch across Hampshire and Sussex, are classed as environmentally sensitive.

But a particularly vigorous type of grass has been rapidly smothering out less competitive plants like rockrose, cowslip and salad burnet.

Tor-grass has reduced the number of insects on the grasslands, and the forage value of the grassland as very few livestock will eats its tough foliage.

'Certainly worked'

But one animal that does graze effectively on the grass is the Exmoor pony.

Two years ago Monty Larkin, from the South Downs Conservation Board, brought a herd of Exmoor ponies onto the chalk grasslands, helped financially by English Nature.

Since then there has been a significant reduction in the amount of tor-grass on the South Downs.

He said: "It's certainly worked. The flowers have increased and the land is suitable once more for sheep and cattle in the autumn."

The government's environment department is hoping the scheme's success will lead to others across the UK.