Mr Benn said the new protected area varied slightly from original boundaries drawn up in 2002, but were in line with the planning inspector's recommendations following a 19-month public inquiry.
The Western Weald, Lewes and the village of Ditchling are inside the park's boundary, and further additions depend on consultation, he added.
Some people have been waiting for this for over 60 years
He said: ""It is fitting that, in this year, the 60th anniversary of the radical legislation that gave birth to National Parks, we are celebrating an addition to the family.
"National park status can be a real boost for the local economy, attracting new visitors, businesses and investment, but above all, the South Downs' wonderful countryside will be protected forever for the enjoyment of everyone."
The decision has been branded "profoundly undemocratic" by Eastbourne Conservative MP Nigel Waterson who said it could affect plans to improve the A27.
"Of course a National Park can bring some benefits, including tourism.
"But ministers have ignored the views of almost all local authorities in the area, as well as local people.
Richard Harris: "It's got a great intimacy as well as the broad sweep"
"I have two major concerns affecting my constituents. First, the extension of the boundary of the proposed park to include Lewes could put a block on long-overdue improvements to the A27.
"Also, there is a real worry that local planning applications will be taken out of local control and decided by an unelected quango."
Defra said the national park designation order could not be confirmed until the further consultation had finished and a proper decision had been made on additional areas.
That probably meant the park would not be formally created until early in 2011, a statement added.
The South Downs National Park is the first to be created in England since the New Forest in 2005.
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