Admiral of the Fleet Sir Geoffrey Thomas Phipps Hornby GCB was a Royal Navy officer. As a junior officer, he saw action at the capture of Acre in November 1840 during the Egyptian–Ottoman War. As a captain, he was assigned to Vancouver Island with a naval brigade where he found a unit of United States troops ready to take over the San Juan Islands in a dispute that became known as the Pig War. Hornby used his powers of diplomacy to facilitate a peaceful handover of the islands to the United States.

Admiral of the Fleet is a five-star naval officer rank and the highest rank of the Royal Navy, formally established in 1688. The five-star NATO rank code is OF-10, equivalent to a field marshal in the British Army or a Marshal of the Royal Air Force. Apart from honorary appointments, no new admirals of the fleet have been named since 1995, and no honorary appointments have been made since 2014.