Britain considered attacking Tokyo with chemical weapons almost a year before the US bombardments that ended World War II in Asia, declassified records revealed on Friday.
The records at the National Archives, which have remained hidden from public view for 65 years, revealed clear proposals to use gas on civilians in 1944.
Although the plan was never put into operation, a detailed memorandum laid out measures to ensure any attack would have the most devastating impact possible.
A Chemical Board note marked "secret" and signed by E.E. Haddon, Secretary, stated: "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.
"Particulars of the population and layout and photographs of typical buildings and areas in Tokyo were kindly provided by the Director of Military Intelligence, War Office and those have now been studied by Professor Brunt."
Blunt, in a memorandum attached to the document, suggested the initial bombardments should take place in areas of densely packed buildings, using incendiaries "sufficient to set the large areas involved on fire."
Once the inflammable buildings of the Japanese capital have been destroyed, he suggested, a gas attack on the "more modern type of streets" could begin.