G.P.O. / BT Defence Network Organisation

G.P.O. / BT War Organisation (BTWO)

I am most grateful to Dave McKay G1JWG ( www.radiohistory.uk ) for his help in highlighting this subject matter, which he spotted in the contents of declassified Cabinet Office file CAB134-5766, held in The National Archives at Kew.
Extracts from a Home Office report dated 25th May 1993 to be considered at a Cabinet Office meeting on 10th June 1993.
14. The BTWO which would have been dispersed to protected sites, was intended to facilitate BT's restoration of damaged communications in time of war. It was interlinked by their bespoke communications system, the BT Defence Network (BTDN). The GREC recommended that the BTWO be maintained, and its size and arrangements reviewed. A review was initiated by the PRWP in 1989, but suspended pending the outcome of BT's Project Sovereign re-organisation. A joint BT/Home Office working group was then set up by the PRWP in early 1991 to carry forward the review.
15. Once again, the changed political climate and diminished threat of nuclear war were major factors taken into account by this review. Other significant considerations were: the enhancement of the ECN and closure of the EMSS, BBR and AUG; the updated requirements of other Departments; the absence of comparable facilities for other essential services; and the high cost of modernising the BTDN. In July 1992 the PRWP accepted the Home Office's conclusion that the BTWO should be considerably reduced in size to a central core, but that BT's designated wartime Network Management Centre at Oswestry should be retained. The closure process is continuing, and the BTDN was shut on 28 February 1993. The Home Office will pay for the cost of BT's connection to the ECN.

Abbreviations Used

ECN Emergency Communications Network, a government voice network, during the Cold War period linking Regional Government HQ, Local Authority Emergency Centres (LAEC) and UKWMO Groups and Sectors. Post Cold War, after the closure of the RGHQ and UKWMO sections, an enhanced ECN was extended to link together the LAEC with many other user groups. The ECN is described in more detail on its own page; Please select from the Chapter Index in the top of page menu bar, scroll to UKWMO and RGHQ Communications Networks then locate Second Generation Speech and Telegraph Network.
GREC Government Review of Emergency Communications.
PRWP Post Review Working Party - implementing the GREC Review.
The next three abbreviations are systems operated by BT, each has a subject page of their own on this website. Please select from the Chapter Index in the top of page menu bar, find these chapters by scrolling to the section GPO / BT : Home Defence Communications

G.P.O. / BT Structure

G.P.O. Telephones, up to and for a number of years beyond privatisation, was organised with Telephone Headquarters in London setting the company policy and budgets. The country was further divided into Regions who were responsible for their part of the budget and exchange and lineplant planning. Each Region was further subdivided into Areas. Each of these areas had a 'Telephone Manager' to who the various departments were accountable. The majority of these departments were located in a 'Telephone Managers Office' a large building or collection of buildings housing hundreds of staff. The map below shows the Area boundaries and the location of both Area and Regional Headquarters.
Telephone Areas within Regions
GPO Regions and Areas
The Post Office War Organisation (POWO) was organised to reflect these Regional and Area boundaries. A national emergency centre was located in Oswestry coordinating the response.
Post Office telephones was privatised in August 1984 and the POWO became the British Telecom Defence Network (BTWO) In the years after privatisation, during the 1980s there were frequent structural changes. Regions were abolished and areas amalgamated to create Districts. Later those combined to form larger districts almost the same size as Regions had been, but without the bloat of a regional organisation between them and headquarters. Fearing the Government would split the company into smaller units, some departments moved into a 'National Networks' organisation (NN) and the remainder in 'Local Communications' sector (LCS).
The communications network to support the POWO was known as the Post Office Defence Network (PODN) becoming the British Telecom Defence Network (BTDN) after privatisation.
The BTDN at the time of its closure in the nineties contained a mishmash of various circuit markings on switchboards reflecting the original PODN and the various company reorganisations during its life as BTDN. This makes identifying the underlying structure difficult for the researcher.

Area War Group

Reflecting the GPO's Regional and Area structure when the war organisation and its communications network were established in the nineteen sixties, each Telephone Area had an area war group (AWG). This was responsible for co-ordination within its area and liaison with Emergency Service customers. One role was to draw up a list of lines with enhanced telephone service preference, known as 'Preference Category I' and 'Preference Category II' and supply this to the Engineering staff to ensure customers lines terminated on the appropriate type of exchange equipment.
GPO/BT Telephone lines have three categories of preference. Ordinary domestic and most business customers are Category III. Important (to the functioning of the state) business lines and Public Telephone Boxes are Category II. Important customers such as Police, Fire and Ambulance stations, Hospitals, Civil Defence and Military have a Category I status. Introducing category two, prevents category three customers from making calls. Introducing category one, additionally prevents category two from originating calls. All three categories can receive calls.


The switchboard at a stand-alone AWG is the common type of office / factory switchboard. Known within GPO/BT as a 'Switchboard N1070' this was modified to diagram number SA2054 for use in small EMSS exchanges and here in an AWG. The 'Switchboard AT3796' designed in 1935 has an almost identical appearance to the untrained eye to this superseding N1070 that was still being installed in customers premises in the early nineteen seventies.
AWG Telephone Switchboard and Interface Circuits
Stand alone switchboard
Typically the AWG switchboard would have a small group of ex-directory telephone lines, lines to adjacent Telephone Area's AWG, A Trunk Subscriber (TkSub) to the EMSS and Private Wires (PW) or Emergency Circuits (EC) to its Regional BTDN command centre. An emergency circuit is a private circuit that is switched through in an emergency or war situation, but in peacetime carries normal calls between exchanges.
As well as telephonic communications, the AWG had access to the public TELEX system and a private BTDN Telegraph network for exchanging typed messages. In this photograph the left hand teleprinter is on the TELEX network and the other on the BTDN system.
AWG Teleprinters
Two Teleprinters
The two devices here are the British Telecom Cheetah, known as a 'Teleprinter 87C' and probably having 32 kilobytes of memory for composing messages. Introduced in the early eighties, the Cheetah's life was short lived due to the influx of personal computers and email. The BT public TELEX Network was finally closed in March 2008.

The Defence Network

The Post Office Defence Network renamed the BT Defence Network (BTDN) used manual switchboards to link important operational buildings, such as larger telephone exchanges, repeater stations and microwave radio stations with coordination points in the wartime management structure of Area War Groups (AWG) and Regional War Groups (RWG). When post-privatisation reorganisations removed the regional tier of BT's peacetime management structure, the RWG were replaced with a number of control centres around the country. Two have been identified so far, Evesham and Skipton, but there must have been a number of other centres.
At the majority of sites the BTDN switchboard would be combined with the Emergency Manual Switching System EMSS. In other towns the BTDN switchboard was completely separate either located in nearby rooms or in completely different buildings, such as Chester and Worcester.
The BTDN part of the combined switchboard generally has very few circuits. One circuit to the AWG and one each to the doorkeeper's red telephone at a handful of larger local exchanges. These are marked 'XX SEC' where the 'XX' is an exchange traffic division code. The 'SEC' circuit from the doorkeeper's lodge in the building containing the BTDN / EMSS switchboard is connected to another location never its own switchboard. Sometimes there are circuits to important Telephone Repeater Stations (TRS) and automatic exchange fault coordination points.
At a combined BTDN and EMSS, the BTDN circuits are not used for connecting customers, but the EMSS circuits may be used to connect BTDN services through to departments in other parts of the country.
BTDN on EMSS Switchboard
switchboard jacks
The Cabinet Office file refers to the BT Wartime Network Management Centre at Oswestry, this is co-located with the Network Operations Centre as it was known then. Located at Park Hall, Whittington, SY11 4TB Lat / Lon 52.871399, -3.030835

War Emergency Procedure

Alex, a non-BT person discovered a wall chart in what he thought was a secret bunker at Abergavenny telephone exchange, this is reproduced below. Abergavenny along with all large exchanges built post-WWII have protected accommodation in their basement which might seem like a bunker to the uninitiated.
Abergavenny is thought to have once been the wartime command centre for the telephone region. The chart refers to BT Instruction numbers, but those are not to hand. Unfortunately some actions make little sense without seeing the details they refer too in the instruction.
Action Notes
EM 001Review plans 
EM 111-2Designate staff 
 Review transport 
 Review essential records 
 Review access arrangements 
 Finalise Instructions 
EM 112-1Alert advance staff 
 Prepare passes 
EM 113-1Alert EM Staff for ZHQ10
 Confirm transport 
 Despatch advanced staff 
 Alert common services 
 Alert EMSS Staff4
 BTDN Ready1
 Provide funds 
EM 114-2Despatch common services 
 Erect F.O.P. 
 Engineering Stores (BTDN)1
 Alert Main Staff 
 Despatch EM Staff to ZHQ10
 Provide transport 
 Man BTDN1
 Advise BTHQ2
 Despatch main staff and records 
 Despatch ZHQ Staff10
 Advise BTHQ2
EM 116Issue instructions 
EM 131Review non-essential work 
 Review essential supplies 
 Review dispersal 
 Review leave 
EM 134Cease training 
 Cease planning 
 Confine maintenance 
 Disperse supplies 
 Top-up fuel 
 Cancel leave 
 Advise BTHQ2
Action Notes
EM 135-2Requisition 
EM 27Provide Comms 
EM 803Prepare staffing schedule 
 Alert staff 
EM 805Switch ECs9
 Provide Staff 
 Review access arrangements 
 Finalise instructions 
EM 811-2Review KPs and Protect6
EM 812-1Complete protection 
EM 831-1Review RADIAC8
 Designate BTLOs 
EM 832-1Alert BTLOs3
 Collect RADIAC8
EM 835-1Despatch BTLOs3
 Distribute RADIAC8
 Advise BTHQ2
EM 852-2Control News Releases 
EM 862Switch BBC ECs9
EM 871Review plans 
EM 874-1Erect F.O.P.5
 Make EMSS Ready4
 Confirm staff alerted 
 Introduce PREF CAT II.7
 Prepare for PREF CAT I. 
 Advise BTHQ 
EM 874-2Prepare LR 
EM 875Activate EMSS4
 Introduce PREF CAT I.7
 Advise BTHQ2
EM 876Activate LR 
 Advise BTHQ2

Author's notes

These do not form part of the wall chart, but added to explain some details. The use of BT implies its predecessor the Post Office / G.P.O. too.
  1. BTDN BT Defence Network, telecoms to support the work of the BT War Organisation in the event of the normal public network failing.
  2. BTHQ British Telecom war headquarters.
  3. BTLO BT Liaison Officer. An engineer embedded within a UKWMO, regional government or council bunker to maintain its communications systems.
  4. EMSS Emergency Manual Switching System, a skeleton manually operated telephone system supporting the military and civil defence organisations. This is described in it own chapter found in the Chapter Index on the page menu bar.
  5. F.O.P. Fall Out Protection, special protection often in the form of dry interlocking construction blocks used to create a wall, to shield staff from the radiation given off by nuclear fallout.
  6. KP Key Point, important parts of the telecoms network requiring protection from sabotage during the period of tension. Protection provided by the police and military.
  7. PREF CAT abbreviation commonly used for Preference Category. The three categories are explained in the Area War Group topic.
  8. RADIAC Devices for measuring the radiation levels after a nuclear attack to see where was safe to go when clearing faults or attending work.
  9. Switch EC, Emergency Circuits (EC) are normal lines between exchanges during peacetime carrying our calls around the country. At times of war and exercises these lines are switched to create private circuits for special services. Prior to the nineteen eighties communications modernisation, the UKWMO relied heavily on ECs. The BBC ECs, would carry special War Time Broadcasting Service programs to their transmitters all over the country from a central underground studio at Wood Norton, Worcs.
  10. ZHQ Zone Headquarters, formerly known as RSoG (Regional Seat of Government) or RSG and RGHQ (Regional Government HQ) At times of war, GPO/BT Engineering Maintenance (EM) staff would be stationed within the ZHQ buildings to maintain and clear faults on telecoms equipment.
Creative Commons LicenceThis page is Copyright © RINGBELL.CO.UK, under a Creative Commons License details are explained here.