The Downs, which cover parts of Sussex and Hampshire, were among 12 areas identified for national parks in the 1940s.
The area spans a tract of land measuring about 100 miles from Eastbourne in East Sussex to Winchester in Hampshire. Within the Downs are two Areas of Outstanding Natural Beauty – the East Hampshire AONB and the Sussex Downs, now covered by the South Downs Joint Committee.
Some 85% of the new park area is farmed, making it unlike the “wilderness” areas covered by existing National Parks.
DEFRA secretary Hilary Benn said the area would become England’s ninth national park.
The announcement means the area will be given the highest level of protection under the planning system.
But shadow DEFRA secretary Nick Herbert, who is MP for Arundel and South Downs, said: “I feel strongly about this special area and I want the best possible arrangements to ensure that its unique and precious landscape is protected.
“But this is the wrong decision which has been rushed for political reasons. The government is making this announcement before it has even determined the final boundaries of the Park.
“It would have been better to have strengthened the South Downs Joint Committee, which is working perfectly well to manage the existing Areas of Outstanding Natural Beauty,” said Mr Herbert.
Unlike other national parks, some 85% of the South Downs park is farmed.