William Francis Forbes-Sempill, 19th Lord Sempill, AFC, FRAeS,[1] (24 September 1893 – 30 December 1965) was a Scottish peer and record-breaking air pioneer, who was later shown to have passed secret information to the Imperial Japanese military before the Second World War.

He was eventually forced to retire from the Royal Navy in 1941, after being discovered passing on secret material to Tokyo shortly before Japan attacked Pearl Harbor and declared war on the United States and the British Empire.

[5] At the outbreak of war, Sempill joined Royal Flying Corps, being granted a probationary commission as a second lieutenant on 15 August 1914,[6] which was confirmed less than four months later.

[8] The following year, in February, Sempill took up a position as an "experimental officer" at the Central Flying School,[9] and he received a promotion to lieutenant in April.

[16] On 1 April 1918, with the amalgamation of both flying services into the Royal Air Force, Sempill was transferred, and was appointed one of several deputy directors in the RAF's personnel department with the temporary rank of colonel.

[4] On 4 September 1930, he set a new record by flying a de Havilland DH.80 Puss Moth seaplane (G-AAVB) 1,040 miles non-stop from Brent Reservoir in London to Stockholm, Sweden, in 12 hours.

[21][22] In early 1931, Sempill was appointed chairman of National Flying Services, the government-subsidised owner and operator of London Air Park, Hanworth.

"[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.

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.

[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 for violating the Official Secrets Act.

[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.

[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.

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.

[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.

[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.

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.

"[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.

[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.

Their second daughter, June Mary, was born in 1922, and was killed aged 18 as a result of enemy action on 11 May 1941 – the last day of the Blitz – at 15 Basil Street, London.

[42] In 1944, Lord Sempill called on the Premier of Nova Scotia, Alexander Stirling MacMillan, in Halifax, and he offered to buy a part of the province.

The National Archives states that "on the evidence of these [1920s] files", Sempill's activities on behalf of the militaristic Japanese and Fascist contacts were less from any desire to help the enemy but more motivated by his own impetuous character, obstinacy, and flawed judgement.

[30] In correspondence of the early 1940s, between Churchill's office, the Attorney General and the Director of Public Prosecutions, it is observed that Sempill had debts and an overdraft in excess of £13,000 (equivalent to £750,000 in 2012).