Colossus – the first ever programmable electronic computer was unveiled at Dollis Hill on 8 December 1943 and then dismantled and rebuilt at Bletchley Park in January 1944.

During the Second World War, Thomas “Tommy” Flowers MBE, son of a bricklayer, was working as senior engineer for the General Post Office Engineering Department at its Research Station in Dollis Hill, north-west London. The GPO Engineering Department would eventually evolve into today’s BT.

In February 1941 he took a call from Alan Turing, the codebreaker at Bletchley Park. Turing needed a decoder for the “relay-based Bombe machine” which he had developed to help decrypt the German Enigma codes.

London base: the Research Department (part of the GPO Engineering Department) was based in Dollis Hill Credit: BT Archive

Although the project eventually folded, Turing was so impressed with Tommy that he introduced him to Max Newman, the man leading the wartime effort to break more sophisticated German ciphers, in particular the coded messages used by the central German High Command, which were created by the Lorenz encryption device.

Tommy proposed an electronic system that was to become, in effect, the earliest digital programmable computer. While the previous most complicated electronic device had deployed only 150 valves, Tommy’s device – born from his pre-war research into early electronic telephone switching – used more than 1,800.

Changing times: Tommy Flowers (in 1980) with electronic computer semi-conductors, early thermionic valves and a modern microchip equivalent Credit: BT Archive

Such ambition meant some of the experts at Bletchley Park were sceptical as to its reliability. Tommy was encouraged to proceed on his own, which he did, with a small GPO team around him at Dollis Hill, using GPO telecommunications equipment and parts and providing much of the funds for the project himself.

Mark 2 Colossus went into service on 1 June 1944 and immediately produced vital information for the D-Day landings to proceed on 6 June

Eleven months later he unveiled Colossus – so named on account of its size. Prototype Mark 1 was demonstrated on 8 December, 1943 at Dollis Hill, and it was evident that the innovative use of electronic parts reduced the time for the huge amount of mathematical work that needed to be done to hours, rather than weeks. Colossus was then dismantled and reassembled at Bletchley Park in January 1944. Tommy was 38.

Eager to build on his success, a Mark 2 design using 2,400 valves was begun before the Mark 1 Colossus was even finished. Mark 2 went into service on 1 June 1944, and immediately produced vital information for the D-Day landings to proceed on 6 June.

Decryption centre: Bletchley Park was the main site for codebreakers during the Second World War Credit: BT Archive

All 10 Colossi machines used during the Second World War were eventually dismantled – and two more – Colossi 11 and 12 – were moved to GCHQ’s headquarters where they may have played a significant part in the codebreaking operations of the Cold War. These last two working Colossi were finally decommissioned in 1959 and 1960.

After the Second World War, Tommy helped design the world’s first all-electronic telephone exchange at Highgate Wood, which opened on 12 December, 1962. This milestone paved the way for the world’s first digital exchange, the Empress Exchange, which opened in West Kensington in 1968.

Back together: Tommy Flowers at a team reunion at BT’s research labs at Martlesham, Ipswich in 1981 Credit: BT Archive

To celebrate Tommy’s contribution to telecommunications, a life-size bronze portrait bust of him now stands at BT’s Adastral Park research laboratories and campus near Ipswich. He is also commemorated in the recently opened Tommy Flowers Institute at Adastral Park, which brings together ICT-sector organisations with academic researchers to solve some of the challenges facing UK businesses, exploring areas such as cyber-security, big data, autonomics and converged networks.