British officials considered attacking Tokyo with poison gas in 1944, more than a year before the United States dropped two atomic bombs on Japan.

Documents made public today include a memorandum written by a government academic entitled Attack on Tokyo with Gas Bombs. His report was coupled with a note from the Ministry of Supply, dated May 22, 1944.

It said: “In his report on his discussions in America Major-General Goldnoy suggested that it might be worthwhile attempting to assess the probable effects of a C.W. [chemical weapons] bombing attack on Tokyo.” A two-page analysis of such an attack was written by Professor D. Brunt, based on information and photographs of the Japanese capital provided by the director of military intelligence at the War Office. He listed two gas options — phosgene and mustard gas — and considered incendiary bombs as well.

The professor discussed the pros and cons of attacking Tokyo in the winter or the summer, as well as how to cause maximum deathsby bombing a crowded neighbourhood rather than a more open area.

“The winter is on the average cold, and may be so cold that the danger from mustard gas would be negligible,” he wrote in his note, dated May 8, 1944. The hot summer was a better option, provided that the attack took place during a gap in the heavy rain that typically occurred.

He noted that many buildings in Tokyo were wooden and easy to ignite. Residential areas had narrow streets, whereas official buildings were on wider, European-style roads.

“In the densely built areas of Japanese-type buildings, where the streets are narrow, the flow of a gas cloud would be hindered by the narrowness of the streets,” he concluded. To get around the problem, Professor Brunt recommended using “very large numbers of small bombs”.

He also suggested phosgene in possibly a 50lb bomb. That would “undoubtedly produce casualties in considerable numbers, as would mustard sown thickly over the area, especially during intervals in the summer rains”. If people fled their houses after a gas bombing, “attacks with incendiaries should follow a few days later”.

The best technique, however, would be incendiaries. “It is therefore recommended that the initial attack on Tokyo should be made against the areas of densely packed Japanese buildings using incendiaries on a scale sufficient to set the large areas involved on fire,” he said. “When the inflammable buildings had been eliminated, attack on the modern-type streets would be made with persistent or non-persistent gas much as against any European city, on the scale already laid down for the latter.”

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