Q-Whitehall is the (possibly unofficial) name given to a communications facility under Whitehall.
The facility was built in a 12 ft (3.7 m) diameter tunnel during World War II, and extends under Whitehall from Trafalgar Square to King Charles Street. The project was known as Post Office scheme 2845. A detailed description, with photographs, was published just after the war in the January 1946 edition of the Post Office Electrical Engineers Journal.
Sites equipped with unusual amounts of GPO/BT telecommunications plant are given a BT site engineering code. This sites code was QWHI, and this is presumably the origin of the name Q-Whitehall.
The site provided protected accommodation for the lines and terminal equipment serving the most important government departments, civil and military, to ensure the command and control of the war could continue despite heavy bombing of London.