New visitor facilities will be built at the restored Wrest Park
A Grade I-registered landscape in Bedfordshire is to be transformed under a 20-year plan.
Preservation work began last year at Wrest Park, near Bedford, which English Heritage said had remained neglected and untouched for decades.
English Heritage said the unaltered park presents a rare "who's who" of 18th Century garden designers including Capability Brown and William Kent.
The gardens and Grade I listed mansion will now be fully restored and opened.
The 380-hectare (939-acre) gardens date back to 1710 and are a rare example of combining formal garden features with informal lakes.
Simon Thurley, chief executive of English Heritage, said: "Wrest Park and its outstanding collection of historic garden buildings is one of the finest surviving 18th Century landscapes on Britain.
"Few knew of its magnificence as for many years it was part of an agricultural research institute.
"Wrest Park is now being restored, reversing years of neglect and placing this once famous and beautiful landscape back into the limelight."
The restoration will see lakes and fountains restored, a Swiss cottage dating from 1856 re-built and new visitor facilities constructed.
Wrest Park has been called Britain's 'largest secret garden'