HMS Britannia was a 120-gun first-rate Caledonia-class ship of the line built for the Royal Navy. Completed in 1820 she was not commissioned until 1823 when she became a flagship at Plymouth. The ship remained there until 1829 when she became flagship of the Mediterranean Fleet. Britannia was often a flagship wherever she was assigned. She became a private ship in 1852 in the Mediterranean Fleet and participated in the Bombardment of Sevastopol during the Crimean War of 1854–1855. Britannia returned home the following year and was converted into a hospital ship. She became a cadet training ship in 1859 and was broken up ten years later.