During World War II, he worked at Britain's secret decryption centre at Bletchley Park, where he was one of the most important contributors.

GCCS established a centre ("Station X") for decryption and analysis of enemy (mostly German) encrypted messages at Bletchley Park (BP).

Such metadata analysis can reveal a lot about enemy organisation, movements and activities, even when the content of the messages remains unknown.

Welchman became head of Hut Six, the section at BP responsible for breaking German Army and Air Force Enigma ciphers.

[citation needed] As head of Hut Six, Welchman was also closely involved in other work which yielded breaks into Enigma by taking advantage of German operational weaknesses and lapses.

[4] His team of young women included Ethel Houston, who would later become the first woman to be made senior partner at a Scottish law firm.

His main interest at this time was the development of similar machines for attacking more advanced German ciphers, such as the Geheimschreiber.

[8] After the end of the war Welchman took up Hugh Alexander's old post as director of research for the John Lewis Partnership.

[13] Welchman died in 1985; his final conclusions and corrections to the story of wartime code breaking were published posthumously in 1986 in the paper "From Polish Bomba to British Bombe: the birth of Ultra" in Intelligence & National Security, Vol 1, No l. The paper was included in the revised edition of The Hut Six Story published in 1997 by M & M Baldwin.

[13] The programme was entitled Bletchley Park: Code-breaking's Forgotten Genius[15] and as The Codebreaker Who Hacked Hitler when broadcast on the Smithsonian Channel in the US.

On 26 September 2016, a blue plaque was unveiled by his daughter, Susanna Griffiths, at St Mary's Church, Fishponds, in Bristol.

Speaking at the event, the Director of GCHQ Robert Hannigan acknowledged the harsh treatment of Welchman, and paid tribute to his "immense contribution" as a "giant of his era".