United Kingdom aircraft registration is a register and means of identification for British owned and operated commercial and private aircraft, they are identified by registration letters starting with the prefix G-. Registration An aircraft registration is a unique alphanumeric string that identifies a civil aircraft, in similar fashion to a number plate on a vehicle. The letter Q has not been used since the issue of G-EBTQ in 1927 (although a few historic aircraft still maintain registrations with this letter), and the CAA also disallows combinations that may be offensive. In accordance with the Convention on International Civil Aviation all aircraft must be registered with a national aviation authority and they must carry proof of this registration in the form of a legal document called a Certificate of Registration at all times when in operation. The register is maintained by the United Kingdom Civil Aviation Authority. Temporary register 1919 Although aircraft had been flown in the United Kingdom since 1908 they were not registered and were not required to carry any external markings. The principle of registering and externally marking aircraft had been agreed upon in 1910 at an international convention in Paris and a draft Air Navigation Order had been prepared in 1911, but was not put into force before the First World War. With the end of the First World War the Air Navigation Regulations came into force on 20 April 1919, allowing civil flying to commence on 1 May 1919. The regulation stated that all aircraft must carry registration marks and any passenger-carrying aircraft must have a certificate of airworthiness. The International Air Navigation Conference in Paris had not concluded, so a temporary system was started; former military aircraft would retain military serials and any new or re-built aircraft would be registered in a sequence starting with K-100. The first civil flight under the new regulations took place on 1 May 1919 when a de Havilland DH.9 of Aircraft Transport and Travel flew from Hounslow to Bournemouth using former military serial C6054 as an identity. The "K series" registrations allocated were in the range K-100 to K-175, with K-169 being the highest known to have been used. First permanent register 1919-1929 The Paris International Conference had allocated the British Empire the prefix G followed by four other letters. The United Kingdom decided to use the G-Exxx sequence for heavier-than-air aircraft (for example G-EAAA) and "G-Fxxx" for lighter-than-air