British officials believed Japan would covertly monitor their discussions and activities at the Group of Seven summit in Tokyo in 1979, according to recently declassified documents.
Security experts warned the Japanese could be recording information via the equipment supplied to the British delegation's office, located on the grounds of the neo-baroque state guest house Akasaka Palace, a minute marked 'secret' reveals.
Nancy Deeves, an official at the Foreign Office's protocol and conference department, wrote to the 'PS/PUS' -- abbreviations referring to the private secretary to the Foreign Office's Permanent Under-Secretary, Michael Palliser -- outlining her concerns in advance of the trip.
On June 18, she wrote, "Technologically speaking, for this visit we are dealing with the most advanced nation in the world who have the money to implement their requirements.
"In our delegation offices...there is no speech protection whatsoever. The only means of ensuring it would...be politically unacceptable, involving our CTSD (communications technical services department) officers tearing walls and ceilings apart beforehand, making a continuous check on telephones, typewriters, photocopiers etc...and requiring a much larger RMP (Royal Military Police) contingent to have to mount a 24-hour guard thereafter.
"Without these drastic measures all speech, use of electric typewriters (provided by the Japanese) and photocopier and most, if not all, use of manual typewriters can and, I am advised, almost certainly will, be recorded."
Prime Minister Margaret Thatcher, Foreign Secretary Peter Carrington and Chancellor of the Exchequer Geoffrey Howe all attended the two-day summit at Akasaka Palace.
Deeves feared that while at the delegation office, just a short walk from the palace, officials could have been dealing with highly sensitive Cabinet and intelligence matters unconnected to the summit.
She said the delegation would be asked to keep all classified papers "under their eye" during the day.
The documents would then be removed at night to private offices in the New Otani Hotel, where the three ministers were staying, and kept under Royal Military Police guard.
Deeves writes, "The question of photography of documents from unseen cameras is doubtless one of the items which security department...has in mind."
Deeves also said protecting classified information would be easier in the hotel, as officers from CTSD could "monitor sensitive speech" and check typewriters and photocopiers beforehand.
However, she noted, "They can of course do nothing about telephone conversations."
She wrote that in reality, the only secure area for secure communication in Tokyo was at the British Embassy. The security department also advised that secretaries only write about secret and top-secret material at the embassy.
She recommended warning all private secretaries, duty clerks, secretaries and advisers traveling to Tokyo about the security risks.
Deeves noted that from her experience of previous overseas tours, there was a tendency for weary ministers and officials to drop their guard and forget they were not in their London offices.
She said, "It seems to me that members of the delegation should on this visit keep in mind throughout the consequences of discussing sensitive matters in our Annexe (delegation) offices and weighing that against the inconvenience of working in the embassy."
Hugo Dobson, a professor of Japan's international relations at Sheffield University who closely follows G-7 and Group of 20 summits, said he could not recall seeing declassified documents discussing surveillance at summits before, although he has long assumed it may take place at international gatherings.
There have been unconfirmed media reports that between 2009 and 2013 Britain and Canada, in partnership with the United States, and separately Russia all spied on delegations at summits they were hosting.
Dobson said, "I'm sure Britain had similar concerns (about surveillance) with regard to visits to most countries. What surprises me is the perception among officials that they were dealing technologically 'with the most advanced nation in the world.'
"It demonstrates how dramatically British perceptions of Japan had changed since the end of World War II."
The 1979 summit on June 28-29 was chaired by Japanese Prime Minister Masayoshi Ohira. Other leaders present included Thatcher, Canadian Prime Minister Joe Clark, French President Valery Giscard d'Estaing, West German Chancellor Helmut Schmidt, Italian Prime Minister Giulio Andreotti, and U.S. President Jimmy Carter.
The meeting's main focus was on energy security and how Western nations could reduce oil imports.
The minute from Deeves was closed from public view until September 2022, presumably on the grounds of protecting international relations.
The original file the minute belongs to was released in 2010 at the National Archives in London.