The South London Downs reserve will be formally designated by Natural England at a ceremony later this month. It extends from Chaldon in the south to Sanderstead and Coulsdon in the north, covering an area of 417 hectares with views across Surrey and into the city centre.
The reserve will become the second largest in the capital after Richmond Park. The third is Ruislip Woods, the largest woodland in London. There are more than 220 across the country, the latest being declared earlier this week near Carlisle.