Fishing boats and other small craft sailing the waters around the British Isles were first required to register towards the end of the 18th century.  The need arose because because the coastguard authorities wished to be able to identify vessels at a distance.  This allowed the coastguard to track and log vessels as they moved around the coast, making it easier to locate vessels reported as lost or missing.  It also facilitated import control and action against smuggling.

Vessels were required to be marked on either side near the bow in large clear lettering, normally black on a white rectangular background.  The registration codes are based on the port of registration, where possible being made up of the first and last letter of the name of the port concerned.  The letter-code was augmented with a serial number allocated by the port authority.  “First class” (larger) vessels displayed the letters first followed by the number, whilest “second class” (medium-sized) vessels displayed the number first followed by the letters.  The smallest (“third class”) vessels, if registered at all, displayed the letters first, their small size ensuring they would not be mistaken for a large vessel.

Registration was first introduced with the passage of the “Ship Registration Act” of September 1786 - at which time the British Isles comprised the island of Ireland and the “mainland” (England, Wales and Scotland), together with the Channel Islands (CI) and the Isle of Man (IOM).  Despite the partition of the island of Ireland in 1922 into the Republic of Ireland (ROI) and Northern Ireland, fishing boat registration continued to apply to the geographical terriory as a whole, so that the list below contains entries for the UK, ROI, CI and IOM.

In 1987, the scheme was standardised under EEC legislation and equivalent codes are displayed in a similar way for vessels operating from ports in all parts of the EEC.

Of note is that the British Isles codes largely reflect the relative importance of individual ports as they were in the 18th and 19th centuries.  Rye (RX), for example, is now a town cut off from the sea, whereas the large modern port of Felixstowe in Suffolk does not have a registration code.  In practice, boats are registered according to the nearest port in the registration list.

Although it was formerly the practice to re-register boats on transfered to a new port, for example following change of ownership, the cost or re-registering is now such many boats retain the original registration throughout their life despite transfer to a new port of operation.