It has been argued that during World War II he worked as a spy for the British MI6, and was instrumental in enticing Rudolf Hess to fly to Britain in 1941.

Finnish leader Field Marshal Mannerheim was careful to keep his distance from Hitler and maintain links to the Western Allies.

[6] The perilous journey was via Bristol (Whitchurch) to Lisbon (Sintra) by KLM DC-3, and then to Geneva via rail across Spain and Vichy France.

The diary of Ulrich von Hassell also records the trip and Borenius' eventual meeting with Carl Jacob Burckhardt, a leader of the International Red Cross.

[8] Harris also interviewed Borenius' son, Peter, who told him that Claude Dansey of MI6 had supplied a cyanide pill that was "the size of a golf ball".

Notwithstanding that question, Burckhardt clearly acted on the information and by 10 March, Albrecht Haushofer, Rudolf Hess's intermediary, was aware of the meeting and what was discussed.

[6] Haushofer had returned to Germany via Arosa in Switzerland, where he visited Ilse von Hassell, staying with her recuperating son at the Hotel Isla.

A year earlier, the British amateur diplomat James Lonsdale-Bryans had used the same location and venue as part of his "semi official" peace negotiations.

Borenius had returned to London by mid-March 1941 (having been refused entry into Italy) and resumed his secretarial duties to the Anglo-Polish relief organisation.

Historian Roger Moorhouse, author of Berlin at War, disputed the claims, stating that "MI6 would have little to gain from luring Hess to Britain.

"[10] Borenius was admitted to St Andrew's Hospital, Northampton, in 1946 and transferred to Laverstock House, near Salisbury, where he died in September 1948.