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On 1 November 1919 the Army signals intelligence department at M11b and the Naval signals intelligence from the Admiralty’s Room 40 combined to become the Government Code and Cypher School, known as GC&CS. Led by Commander Alastair Denniston, and based at Watergate House in The Strand in London, GC&CS comprised just fifty personnel. GC&CS relocated to Bletchley Park for the duration of WW2. Once the war was over the newly re-named GCHQ moved to Eastcote. Between 1950 and 1954 the organisation relocated once more to Cheltenham which remains its primary base. On 1 November 2019 GCHQ celebrates its 100th anniversary.


This Collections Uncovered album was first published on 1 November 2019.

During the interwar period GC&CS focused some of its workforce on the threat from Bolshevik Russia. In the late 1920s Russia began using one-time pads to encrypt their most important communications. The system involved using a sheet of randomly generated numbers taken from a larger pad which would only be used once. The sheets are disposed of after use, so the existence of one-time pads is rare. This example shows a Foreign Office ‘out’ one-time pad which was declassified by GCHQ in 2008.

When war broke out on 1 September 1939, parts of SIS and GC&CS had already moved to their war-time home at Bletchley Park. During this time the organisation grew under the leadership of Alastair Denniston. In his diary from 1940 Denniston, amongst jotting down meetings arranged with the leading figures at Bletchley Park, also notes when his son’s half-term falls and when he’s scheduled to play in a tennis tournament. On one of the final pages in the diary, Denniston has made notes from a meeting. The ‘C’ mentioned here is likely Stewart Menzies who became Chief of the Secret Service in 1939, and thus had overall authority over GC&CS.

Bletchley Park staff began to disperse after VE-Day and VJ-Day. Some BP Veterans continued to work with GC&CS but many went back to civilian life. Muriel ‘Mimi’ Gallilee worked as a copy typist for the Director’s Secretariat. She continued to work with GC&CS until September 1946. Clive ‘Joe’ Loehnis wrote Gallilee several references when she was looking to leave the department. Clive Loehnis would become GCHQ’s fourth Director in 1960. Gallilee would go on to work as a BBC researcher for 22 years.

This draft of an order released on 27 December 1945 indicates that GC&CS were re-organising their workforce. The final arrangements for a permanent move to Eastcote were still under consideration, including salaries and the number of staff to be taken on. The staff employed at Bletchley Park peaked at over 9,000; the cohort which transferred to Eastcote in early 1946 was a much reduced 1,000 employees.

Barbara Start moved to Bletchley Park in March 1945 and was considered for the move to Eastcote in January 1946 to the ‘permanent organisation’. Start was a candidate for Grade CV which was reserved for university graduates. The letter also implies that the offer of employment was conditional depending on the final budgets designated to GCHQ by the Treasury.

Barbara Abernethy, who had joined CG&CS before the outbreak of WW2, worked as Denniston’s personal assistant. Abernethy continued to work for GCHQ after the war at Eastcote. This letter from Alan Rousseau Bradshaw to Miss Abernethy is dated 10th May 1945, just a couple of days after VE-Day. Bradshaw had overseen the administration of GC&CS throughout the war. Here Bradshaw expresses his thanks to Abernethy for her service as he resigns from his post as Deputy Director (2) of GC&CS.

Arthur ‘Bill’ Bonsall was promoted to head of Air Section at Bletchley Park in July 1945 largely in order to prepare for the move to Eastcote. During his time at Bletchley Park Bonsall had been responsible for ‘BMP’ reporting, which took its acronym from the three co-creators; Arthur Bonsall, William Millward and Frederick Seaton Prior. These reports provided summaries of intelligence derived from Luftwaffe voice traffic. This document from June 1944 is Bonsall’s own copy of a BMP report on fighter patrols. Bonsall would become Director of GCHQ in 1973.

After VE-Day, staff who were responsible for specific Sections during wartime began compiling official internal histories which would be retained by GCHQ. Taken from Volume VIII, of 24 volumes on the history of Naval Section, this page details the information gathered on U-Boat U-129. Across 444 pages, all the information gleaned by Bletchley Park on German U-Boats is charted. Here we can see that, along with all the commanders of the vessel, intelligence was also able to pinpoint specific dates of the movement of each U-Boat.

The complexity of terminology used within GC&CS during the war meant that it was necessary to compile an index of abbreviations used in the internal Sigint histories. The index is composed of almost 1,000 cards which include acronyms for role titles within GC&CS as well as abbreviations for enemy forces. This card shows the definition of C.S.S. as Chief of the Secret Service, known as ‘C’.

The experience of Bletchley Park Veterans, whether they continued to work for GCHQ or not, would stay with them over the years. Former colleagues Harry Hinsley and Mimi Gallilee corresponded with each other in the mid-1990s. This extract from a letter to Hinsley from Gallilee mentions that it has been 50 years since she last spoke to him. Having read ‘Codebreakers: The Inside Story of Bletchley Park’, which was edited in part by Hinsley, she felt compelled to get in touch as the book had brought back so many fond memories.

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