There are many sources for squawks in the UK online, but a lot of them seem to be based on older lists/information. So, here's the most up-to-date list direct from the UK's Civil Aviation Authority documentation. We'll try and keep this up to date as the documentation is usually reproduced every month, but changes will be rare.

Squawks can be useful for determining where something has come from or is going to in the case of missing route information. Or they can identify a specific type of mission, in the case of things like NATO Air Policing. The emergency squawks are also listed here, of course, but these will alert live on the table on the tracker website and also hopefully soon into the ADS-B channel on the OARC Discord.

Also remember that each digit is an octal, meaning 0-8, so you'll never see a 9 anywhere. Anchor links are provided in the dropdown menu to each section. There are a lot of accompanying notes that go with this data and these are reproduced here, although some are operational rather than informational:

Note 1: Pilots are not to preselect Mode A code settings for discrete codes until instructed to do so by the appropriate controlling agency.

Note 2: The codes or series annotated * are used for conspicuity, co-ordination or special purposes and, unless procedures have been agreed with UK CAA Safety and Airspace Regulation Group, the Mode A code and associated Mode C pressure-altitude reporting data must be considered unvalidated and unverified.

Note 3: These codes are assigned by Aberdeen for helicopters operating in Northern North Sea Off-shore area and out to 5° West.

Note 4: Ground based transponder equipment.

Note 5: This code is allocated to Transit (ORCAM) UK and is temporarily assigned for use by London Control (Swanwick) FIS. Notwithstanding, it retains all the properties and protection of an ORCAM Transit code.

Note 6: These codes will be assigned to aircraft on air policing missions within the UK FIR/UIR operating under Air Defence Priority Flight (ADPF) status. NATO Air Surveillance and Control System (ASACS) units, NATO Airborne Early Warning aircraft or military ATCCs will normally control the aircraft. The actual controlling unit for a particular flight will be notified by the relevant NATO ASACS unit to the military ATC supervisor whose area of responsibility contains the intended route of the ADPF aircraft. The military ATC supervisor will then inform the relevant civil ATCC. Where any doubt about the controlling agency exists, the relevant military ATC supervisor with the responsibility for the area within which the ADPF aircraft is operating should be contacted in the first instance.

Note 7: This code is assigned by the FISO to aircraft requesting entry into Controlled Airspace for the purpose of joining or crossing the ATS Route network. Pilots shall only select the code when they are within two-way communication with the London FIS. Where available, the Mode C pressure-altitude reporting mode of the transponder should also be selected. If communication is lost or the aircraft leaves the FIS frequency, pilots shall deselect this special purpose code. The assignment of this code does not imply the provision of a radar service and the code and any associated Mode C pressure-altitude reporting data must be considered to be unvalidated and unverified.

Note 8: All Spadeadam squawks indicating flight above FL 50 will be validated and verified by Spadeadam ATC.

Note 9: This conspicuity code shall be used by aircraft operating in an Off-shore Safety Area (OSA) when contact with the Area Air Traffic Service Provider (ATSP) has not been established and no other code has previously been allocated.

Note 10: These codes will be assigned to Unmanned Aeronautical Vehicles operating within an active danger area, which will be validated but may be unverified.

Note 11: Aircraft operating within, and in the vicinity of, EGD123, EGD124, EGD125, EGD126, EGD128 (these are specific danger areas).

Note 12: This conspicuity code shall be used by Unmanned Aerial Systems/Remotely Piloted Aircraft that have lost communications with their remote pilot and are following a pre-programmed Lost Link flight profile.

Note 13: This code may also be used for Blackpool Instrument Training.

Note 14: For use within a 20 NM radius of Land’s End up to 4000 FT AMSL.

Note 15: Codes allocated to HQ Air Command are reserved for the purpose of major Operations or Exercises under HQ Air Command control including national events coordinated under HQ 11 Gp.

Note 16: Codes “On loan to CCAMS” may be reallocated to Special Events (e.g. Royal International Air Tattoo) providing at least two weeks' notice is submitted to *CAA email address removed*.