British officials believed in the early 1980s that Japanese institutions had been "slightly" penetrated by communist intelligence services, according to documents declassified Thursday at the National Archives in London.
The documents from 1983 assert that there were approximately 220 communist intelligence officers working in Japan: 100 for the Soviet Union, 60 for China and 60 for other communist countries.
But despite their presence, British officials felt classified information held by the government — particularly the Foreign Ministry and the old Defense Agency — was relatively safe.
The documents, which appear to have been prepared by staff at the British Embassy, warned that in Japan "there is little concept of exclusivity or confidentiality as understood in the West."
The papers came from a Cabinet Office committee of senior British officials that was assessing how different countries protect classified information and the risk of penetration by hostile states.
They concluded that the British government and defense industry should continue to only share "confidential" information with Japan.
Ken Kotani, an intelligence expert at the National Institute for Defense Studies in Tokyo, said he was surprised by the reported number of Soviet intelligence officers. If accurate, it would mean they far surpassed the reported number of British and American intelligence officers.
"This was at the height of the Cold War. The intelligence officers posed as journalists or diplomats and courted their contacts with meals at restaurants and payments," he said.
Elsewhere, British officials in that era regarded French institutions to have been "partially penetrated" by intelligence services hostile to Britain and decided to restrict the release of information to "confidential" intelligence. By contrast, officials concluded Australia should be given access to "top secret" material.
They recommended South Korea should be allowed only "restricted" information and that there should be no release of classified information to China.