William Forbes-Sempill, 19th Lord Sempill

In 1920, he led a civilian British deputation of former naval airmen to Japan – the Air Ministry and the Foreign Office saw the prospect for lucrative arms contracts with Japan – to help develop aircraft carriers,[24] and to assist the Japanese navy in setting up its new air base,[4][24][25] after the Japanese had bought three Supermarine Channel flying boats. Sempill was well respected within Japanese circles, and received a personal letter from Prime Minister Tomosaburo Kato (1922–1923), thanking him for his work with the Imperial Japanese Navy, which he described as "almost epoch-making."[26]

With the termination of the Anglo-Japanese Alliance in 1921, Sempill should have ended close military contact and discussions regarding naval aviation technology and tactics. However, on his return to the United Kingdom in 1923, Sempill kept in contact with the Japanese Foreign Ministry through the Japanese Embassy in London.

In 1925 Japanese intelligence asked Sempill to obtain secret technical information about the prototype of the Blackburn Iris seaplane.

In 1925, Sempill accompanied a mission of foreign air officials to the Blackburn Aircraft factory at Brough, East Yorkshire. The Japanese had previously asked questions about aircraft being developed. Sempill later asked the same questions, in his official capacity, about the then-secret Blackburn Iris.[24]

The Directorate of Military Intelligence had kept Sempill's communications with the Japanese intelligence officer/Naval attaché in London, Captain Teijirō Toyoda, under surveillance from 1922. This led to the knowledge that Sempill was passing classified information to the Japanese that Toyoda's communications indicated had been paid for.[26] MI5 tapped Sempill's phone, and observed that his servant was a Japanese naval rating.

In March 1926, Sempill was proposed by the Aviation Ministry for an appointment as Greece's aeronautical adviser. At this point, the Directorate of Military Intelligence advised the Foreign Office and the British Embassy in Athens that Britain could not be seen to endorse Sempill's appointment because of his past activities.[26]

Sempill was called into the Foreign Office for an interview. The questions directed to him were intended to assess his loyalty to the British Government, his attachments to the Japanese, and the amount of information that he had passed to the Japanese. During the meeting, the investigating officer could not reveal that the British had broken Japanese codes, and were monitoring the Japanese communications systems.[24]

On a trip to Brough, Sempill had openly talked about the Iris flying boat with the foreign air officials on the train trip from London. This was witnessed by a British Air Ministry civil servant, who reported the incident. Confronted with this information, Sempill admitted that he had broken the Official Secrets Act.[24]

A subsequent meeting, chaired by the Secretary of State for Foreign Affairs Austen Chamberlain, decided it was not in the interests of the British government to prosecute Sempill.[24] Firstly, Sempill's father was then aide-de-camp to King George V; any public trial would be a grave embarrassment to both the Crown and the British establishment. Secondly, a prosecution would have revealed to the Japanese that British Intelligence had cracked the cypher codes of its diplomatic service.[26]

Six years after admitting he had breached the UK's Official Secrets Act, Sempill became a technical and business consultant to Mitsubishi Heavy Industries; from 1932 to 1936, he represented the Japanese company in Europe.[24] He also became chairman and then president of the Royal Aeronautical Society. In this capacity, he advised overseas governments, such as Australia, on the creation of their naval air services.

In October 1933, Sempill was seriously injured in an accident while riding as a passenger in a prototype Dymaxion three-wheel car in the United States. Sempill had been invited, in his capacity as an aviation expert, to review the aerodynamic experimental vehicle at the Chicago World's Fair. As he was being rushed to an airport to catch a plane to Akron, Ohio, to meet the Graf Zeppelin for its return trip from New York to Europe, the Dymaxion was struck by another vehicle, and overturned, killing the driver and injuring Sempill.[27]

On 28 February 1934, he succeeded his father, John Forbes-Sempill, 18th Lord Sempill, as Lord Sempill and Baronet of Craigevar. His wife, who had accompanied him on many of his air tours, died in July 1935.[28]

Sempill had "an affinity with militarist right-wing regimes".[24] During the 1930s, he developed extreme right-wing political opinions, and was active in several antisemitic organizations such as the Anglo-German Fellowship, the pro-Nazi Link organisation, and The Right Club led by Archibald Ramsay.[29][30]

On the outbreak of war in 1939, Sempill was given a position in the Department of Air Materiel at the Admiralty. This gave him access to both sensitive and secret information about the latest British aircraft.[4]

By June 1941, MI5 had intercepted messages between London and Mitsubishi and Field Marshal Yamagata's Tokyo headquarters indicating payments were being made to Sempill: "In light of the use made of Lord Sempill by our military and naval attaches in London, these payments should continue".[4] On further investigation, MI5 suspected that Sempill was passing on top secret information about Fleet Air Arm aircraft. The matter was passed to the Attorney General and Director of Public Prosecutions. Once more, the Attorney General advised against prosecution. On 5 September 1941, Sempill attended a meeting with the Fifth Sea Lord, and was given "a strict private warning".[4]

On Friday 2 August 1940, Special Branch arrested Japanese businessman Makihara Satoru, head of Mitsubishi Shoji Company's London office, and several others on suspicion of espionage, and took them to Brixton prison.[31][32][33][34] On discovering that Makihara was in custody, Sempill telephoned and then called at Paddington police station to assure the police of Makihara's innocence and character.[24] He was released a few days later, on Monday 5 August, due to "insufficient evidence".[31][35]

Sempill was also probably passing on detailed information about the British government. In August 1941, Winston Churchill and President Franklin D. Roosevelt held a meeting in Newfoundland aboard HMS Prince of Wales to discuss the military threat posed by the Japanese.[24] Soon after, communications between the Japanese embassy in London and Tokyo were deciphered by the Bletchley Park code breakers. The decrypted messages were transcripts of the conference notes. When passed to an alarmed Churchill, he called them "pretty accurate stuff".[24] Three months later, more notes from Churchill's personal agenda and inner circle were intercepted as they were being sent by the Japanese Embassy in London to the Foreign Ministry in Tokyo. Privately, Churchill concluded with Anthony Eden that only two men could be the source of such leaks: Commander McGrath or Lord Sempill.[24]

On 9 October 1941, a signed note from Churchill read: "Clear him out while time remains." The following week the Admiralty told Sempill he must either resign or be sacked.

After Sempill made an official protest, Churchill backtracked. The Prime Minister told the Admiralty: "I had not contemplated Lord Sempill being required to resign his commission, but only to be employed elsewhere in the Admiralty."[4] A subsequent note from Churchill's aide Desmond Morton, dated 17 October 1941, outlined the new position: "The First Sea Lord ... proposes to offer him a post in the North of Scotland. I have suggested to Lord Swinton that MI5 should be informed in due course so they may take any precautions necessary."[4]

On 13 December 1941, six days after the attack on Pearl Harbor, Sempill's office was raided. A search revealed secret documents that he should have handed back to the Admiralty over three weeks earlier. Two days later, Sempill was discovered making phone calls to the Japanese Embassy.[24] Despite the evidence of treason in wartime (see Treachery Act 1940), no arrest or prosecution was ordered; Sempill agreed to retire from public office.[4]