A top secret World War II base has been revealed as the site where maps were prepared for a planned attack on Hitler's bunker at Berchtesgaden.
The hall was the retreat of Benjamin Disraeli (NTPL/Matthew Antrobus)
The true nature of the war work carried out at Hughenden Manor, near High Wycombe, Bucks, has only just been uncovered by researchers.
The information was uncovered as they prepared for 60th anniversary VE Day commemorations.
Map makers at the hall also contributed to the Dambusters bombing success.
Researchers found that the map makers drew up plans to attack Eagle's Nest, Hitler's secret bunker at Berchtesgaden.
Until now it was known simply that Hughenden Manor was used by the Air Ministry during the World War II, but exactly what went on there was a mystery.
Work at the manor included plans to attack Hitler's bunker
It was perhaps best known for being the countryside retreat of Queen Victoria's trusted prime minister Benjamin Disraeli who lived there from 1848 until his death in 1881.
The finding that 100 intelligence staff were based there secretly plotting Hitler's downfall was made by National Trust researchers during plans to mark the 60th anniversary of the end of World War II.
Jessie Binns, who works for the trust, which has run the estate since 1947, appealed to local people to come forward with their memories.
She then learned Hughenden Manor was once at the top of Hitler's hit list.
Fliss Coombs, spokeswoman for the National Trust, said a special order issued after VE Day congratulated Hillside staff for helping ensure the success of the Dam Buster raids and the airborne landings on D-Day.
An exhibition at Hughenden Manor, documenting its wartime history, was unveiled on Monday.