Designed by Sir William Rule in 1812, the huge three-decker H.M.S. Britannia was laid down on No.1 slip in Plymouth dockyard in December 1813 and finally launched on 20 October 1820. A first-rate of 2,602 tons b.m. and mounting 120 guns, she was one of the largest ships of her day and measured 205 feet in length with a 53½ foot beam. First commissioned on 19 January 1823, she did alternate spells of duty as flagship in Portsmouth and then with the Mediterranean Fleet based in Malta before returning home to become Guardship of the Ordinary at Portsmouth in August 1850. When the Crimean War began in the spring of 1854, she went with the fleet to the Black Sea where her rocket boats were in action almost immediately at Odessa on 22 April. Flying the flag of Vice-Admiral Dundas, she led the Anglo-French Fleet which bombarded Sebastapol on 17 October 1854, her Master and boat's crew using muffled oars having successfully evaded Russian sentries and taken all the vital soundings the previous day.

The Crimean campaign proved to be Britannia's last sea-going commission and when she returned to Portsmouth, she was laid up for several years before embarking on her new career which was to make her as famous as any ship in the navy. On 1 January 1859 she was re-commisssioned to become the first training ship exclusively for the education of a naval cadets. Moored originally in Haslar Creek, Portsmouth, this berth proved unhealthy so she was moved to Portland; when this anchorage also proved unsuitable, she was moved a third time -to Dartmouth -in September 1863. By 1865 the number of cadets had risen so sharply that the hulk of the old Hindostan was brought round from Plymouth and joined to Britannia by a covered gangway. The sight of these two ships moored together in the River Dart was to become familiar to generations of young naval officers and, even after Britannia was broken up, she was replaced first by another vessel and then by buildings ashore, all of which took her name in order to perpetuate the tradition.