That's not the only definition of High Treason though- "Under the law of the United Kingdom, high treason is the crime of disloyalty to the Crown. Offences constituting high treason include plotting the murder of the sovereign; committing adultery with the sovereign's consort, with the sovereign's eldest unmarried daughter, or with the wife of the heir to the throne; levying war against the sovereign and adhering to the sovereign's enemies, giving them aid or comfort; and attempting to undermine the lawfully established line of succession. Several other crimes have historically been categorised as high treason, including counterfeiting money and being a Catholic priest." http://en.wikipedia.org/wiki/High_treason_in_the_United_Kingdom The problem traditionally was that treason could only be proven with two eye-witness testimonies to the act. During wartime the Treachery Act 1940 created the new felony of treachery to make it easier to prosecute suspects, since it was subject to the same rules of evidence as other common offences, but this was suspended in February 1946- http://en.wikipedia.org/wiki/Treachery_Act_1940 This was replaced by the Treason Act 1945 which abolished the special status of treason, and allowed it to be tried under normal rules of evidence. http://en.wikipedia.org/wiki/Treason_Act_1945 It also used the Treason Act of 1800 to redefine treason as: "The Act provided that in all cases of high treason which consisted of compassing or imagining the death of the King, or of misprision of that species of high treason, where the overt act (or acts) of that species of high treason alleged in the indictment for that offence was the assassination or killing of the King, or a direct attempt against his life, or a direct attempt against his person whereby his life might be endangered or his person might suffer bodily harm, the accused could be, and was to be, indicted, arraigned, tried and attainted, in same manner, and under the same procedure ("course and order of trial") and on the same evidence, as if he was charged with murder." http://en.wikipedia.org/wiki/Treason_Act_1800 Since treason had been given a specific new definition by 1945, that probably explains why spy cases were tried under other legislation.