Banco de dados da Segunda Guerra Mundial (WW2DB)

More than a year before the United States dropped the atomic bombs on Hiroshima and Nagasaki, British officials had reviewed academic evaluations of conducting chemical warfare against Tokyo. The two chemical agents considered were phosgene and mustard gas, and Tokyo was suggested for its temperature in the summer, high population density, and political importance. There were also recommendations on delivery techniques, for example:

As the consideration of poison attacks were revealed by the United Kingdom's National Archives, detailed plans for the usage of darts containing poison were also made public. They were designed to be dropped by Royal Air Force bombers on German troop concentrations. This plan, drawn up in late 1941, predated the chemical attack considerations on Tokyo by more than two years as the British feared a possible invasion of the UK by German troops.

For more information:
Times Online: Britain considered chemical attack on Tokyo in 1944
BBC: WWII poison darts secret emerges