It's derived from the coat of arms of the Cinque Ports, depicting a lion and a ship's stern. Although the coat of arms of each port look similar by having the heads of three lions on the left joined to the sterns of the ancient ships on the right, each can be identified by subtle changes of design.
This is the flag of the Lord Warden of the Cinque Ports:
What were the cinque ports? (from
http://www.battle1066.com)
Five ports on the Kent/Sussex coast. named from the French word "cinque" meaning five. The Cinque Ports were part of England's defences from the 11th century to at least the 15th. The purpose of such ports were for the production of warships for the country. The ships produced were constructed for the state and their makers were given consideration when paying tallage or ship tax. The ports were under the charge of a warden, known as the Warden of the Cinque Ports. Although the significance of the cinque ports is no longer, the title of Warden still exists on a purely honoury basis.
Located along the southern coast of England, the initial ports were Dover, Sandwich, Romney, Hythe and Hastings. These original five Cinque Ports indicated in red were known as the Head Ports. They were later joined by others such as those shown in blue, for example - Located and Rye which were known as Antient Ports. Even though they were officially acknowledged as Cinque ports from the 11th century, their history dates back to the time of Alfred the Great whose ship building program, some may consider to be the forerunner of the Royal Navy. As time went by, the number of cinque ports expanded to over thirty which would have included just about every south eastern coastal ship building village.
Royal Cinque Ports' logo is quite similar.