Operation Candid: Protection of the Royal Family in an Emergency was a Cold War contingency plan of the British Government to evacuate Queen Elizabeth II, the Duke of Edinburgh and other members of the British royal family from London in the event of nuclear war.
The force would be around 1,300 officers and men consisting of a reinforced battalion from one of the five regiments of Foot Guards, a reinforced squadron of the Household Cavalry equipped with armoured cars, a medical detachment, communications vehicles and other support and transport vehicles.
The force was capable of being split into four separate, self-supporting units to disperse key members of the royal family to different locations.
The home secretary and Prince Philip, Duke of Edinburgh, would remain with Queen Elizabeth II so they could hold a Privy Council and appoint a new prime minister if necessary.
[1] In February 2019, media reported that the Civil Contingencies Secretariat had "repurposed" Operation Candid to evacuate the royal family from London in the event of rioting following a no-deal Brexit; however, neither Buckingham Palace nor 10 Downing Street would comment on the report.