Licences bearing unique callsigns are issued to every amateur radio operator. Regulated by the International Telecommunication Union (ITU), the amateur licence callsigns always follow a specific format.
It is this unique callsign that identifies the radio operator while on the air.
Under normal circumstances once issued, amateur callsigns will not be re-issued without good reason, or until a suitable period of time has passed.

“G” amateur licences & callsigns

The “G” series of amateur licences was first issued around 1920, which consisted of “G2” + two letters. The “G” callsigns were issued until 1996 when the current 3 tier amateur licence system was introduced.

The complete sequence can be seen below:
G2 + 2 letters: issued 1920 – 1939
G3 + 2 letters: issued 1937 – 1938
G4 + 2 letters: issued 1938 – 1939
G5 + 2 letters: issued 1921 – 1939
G6 + 2 letters: issued 1921 – 1939
G8 + 2 letters: issued 1936 – 1937
G1 + 3 letters: issued 1983 – 1988 – originally issued as Class B licence
G2 + 3 letters: issued 1920s to 1939. Originally issued as “Artificial Aerial” licence
G3 + 3 letters: issued between 1946 and 1971. Originally issued to amateur radio licence Class A amateur radio licence holders.
G4 + 3 letters: issued between 1971 and 1985. Originally issued to amateur radio class A licence holders.
G5 + 3 letters: originally issued to foreign nationals as a form of reciprocal licence. They were withdrawn and either they used existing home calls with additional UK prefix / callsign, or if applicable they could apply for UK licence.
G6 + 3 letters: issued 1981 – 1983. Originally issued as a class B licence
G7 + 3 letters: issued 1989 – 1996. Originally issued as a class B licence
G8 + 3 letters: issued 1964 – 1981. Originally issued as a class B licence
G0 + 3 letters: issued 1986 – 1996. Originally issued as a class A licence

Notes:

There were no classes as such until 1964 when the UHF Class B was introduced, and all other licence holders then became Class A. Both classes required RAE (Radio Amateur Examination) passes, the difference being a 12wpm Morse test was required for class A.

Class B ‘codeless’ licensees were restricted to 70cms and higher, but the limit was reduced to 144MHz in 1968. Nearly 20 years later, the Class B licence included 70MHz and 50MHz with the full release of 6m in June 1987.

When the licence system was amended in 2003 to no longer require a Morse test to allow HF use, Classes A and B merged together to become the Full licence class.

Foundation licence & callsigns

A foundation licence allows operation of radios on a variety of amateur radio frequencies. The maximum power allowed is 10 watts. The foundation licence was introduced in January 2002 and is the entry level into Amateur Radio with the aim of allowing you to get involved in the hobby as quickly as possible.

M1 + 3 letters 1996 – current. Originally issued as a class B licence.
M3 + 3 letters 1996 – 2018. Foundation licence holders.
M7 + 3 letters 2018 – current. Foundation licence holders.

Intermediate licence & callsigns

This is a half-way step to holding a full licence. Because a more advanced exam is taken the intermediate licence allows access to all the amateur radio bands. More power allowed, but is limited to 50 Watts, except for the 135.7 – 137.8kHz band, which is limited to 1 Watt.
Amateurs holding an intermediate class of licence use a slightly different format of callsign.

2E0 + 3 letters – Issued from 1991 onwards as Intermediate licence. Issued as Novice class A licence from 1991 for use on all Novice allocations
2E1 + 3 letters – Issued from 1991 onwards as Intermediate licence. Issued as Novice class B licence from 1991 for use on Novice allocations above 30 MHz

Full licence & callsigns

The full licence is currently the highest level of amateur licence in the UK. This level of licence also allows the holder to apply for Notice ofVariation (NoV) to operate unattended repeater systems etc.
M0 + 3 letters 1996 – current. Originally issued as a class A licence

Callsign Modifiers

To denote which country of Great Britain the callsign was allocated, a second letter
is added to the prefix.

England doesn’t use a location identifier.

For use during 2016 only, a special location identifier of “K” was issued by OFCOM. Radio Amateurs with a Main station address in Cornwall or The Isles of Scilly were able to apply to use the “K” during 2016. This was be added to their normal callsign, except for intermediates, where it would replace the “E”. E.g. G1XYZ became GK1XYZ and 2E0ZYX became 2K0ZYX.
This temporary special identifier can now no longer be used.

GB prefix call signs

The GB prefix is used for a variety of special ham radio licences ranging from repeaters and beacons to data mailboxes and special even stations. It is possible to tell the use of the station and licence from the format of the callsign.

For contests it is also possible to obtain callsigns consisting of the prefix plus one letter. Any “special” allocation is at the discretion of OFCOM.