The documents illustrate Britain's involvement in the US' decision to carry out the A-bombings of Hiroshima and Nagasaki in August 1945, something not widely known, Japanese news agency Kyodo reported on the 68th anniversary of the bombings.
Quoting recently declassified documents by the US National Archives and Records Administration, it said the British government officially expressed its support for using the new weapon against Japan at the Combined Policy Committee meeting in Washington on July 4, 1945.
According to the documents, British Field Marshal Sir Henry Wilson told the meeting chaired by US Secretary of War Henry Stimson that the British government "concurred in the use of the T.A. Weapon against Japan."
"The Governments of the United Kingdom and the United States had agreed that T.A. Weapons should be used by the United States against Japan, the agreement of the British Government having been communicated" by Wilson, it said.
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Britain's official agreement on the use of atomic bombs came after US President Franklin Roosevelt and British Prime Minister Winston Churchill agreed at their September 1944 meeting in New York that an atomic bomb might be used against Japan when it was developed, the documents show.
Shortly after the July 1945 committee meeting, the United States conducted the first atomic bomb test in New Mexico.
The first operational atomic bomb was dropped by the US on Hiroshima on Agugust 6, 1945. Three days later, another Japanese city Nagasaki was pounded by the atomic bombs.