Following an international convention in Paris in 1910, a draft Air Navigation Order was prepared in 1911 requiring all aircraft to be registered. This did not come into force until after WWI, on 20 April 1919. Regulations stated that all aircraft must carry registration marks and any passenger-carrying aircraft must have a certificate of airworthiness. Although aircraft had been flying in the UK from 1908, a temporary register was started in 1919 using a sequence beginning with K-100. The current register started on 22nd July 1919, when the Paris International Conference allocated the prefix G- followed by four letters for British Empire registrations. Civilian aircraft flying on former military serials and those allocated with the K-100 series were re-registered sequentially using the following blocks:
G-EAAA to G-EZZZ : UK Heavier than air aircraft (1919–1928). Only registrations G-EAAA to G-EBZZ, and G-EDCA were initially issued.
G-FAAA to G-FZZZ : UK Lighter-than-air aircraft (1919–1928). Only registrations G-FAAA to G-FAAX were initially issued.
G-AUAA to G-AUZZ : Australia (1921–1929) susequently replaced by the VH-xxx format.
G-CAAA to G-CAZZ : Canada (1920–1929) susequently replaced by the CF-xxx format.
G-CYAA to G-CYZZ : Canadian government and military aircraft (1920–1928). Only registrations G-CYAA to G-CYHD and G-CYUR to G-CYZZ were issued before being replaced with a numerical sequence consisting of a 3 digit number allocated in blocks depicting aircraft function.
G-IAAA to G-IAZZ : India (1920–1928). Only registrations between G-IAAA to G-IAAY were issued before being replaced by the VT-xxx format.
G-NZAA to G-NZZZ : New Zealand (1921–1929). Only registrations G-NZAA to G-NZAZ and G-NZEA to G-NZEF were issued before being replaced by the ZK-xxx format.
G-UAAA to G-UAZZ : Union of South Africa (1927–1929). Only registrations G-UAAA to G-UABD were issued before being replaced by the ZS-xxx format.
On 30th July 1928 the UK sequence was restarted at G-AAAA, initially with registrations being issued in sequential alphabetical order. This policy was relaxed in the early 1970's allowing personalised markings out of virtually any 4 letter combinations. This led to a number of re-registrations.