[5] The Act was passed at Westminster in the Hilary term of 1351, in the 25th year of the reign of Edward III and was entitled "A Declaration which Offences shall be adjudged Treason".

It was passed to clarify precisely what was treason, as the definition under common law had been expanded rapidly by the courts until its scope was controversially wide.

The Act was last used to prosecute William Joyce, better known as "Lord Haw-Haw", in 1945 for collaborating with Germany in World War II.

Joseph Story wrote in his Commentaries on the Constitution of the United States that: they have adopted the very words of the Statute of Treason of Edward the Third; and thus by implication, in order to cut off at once all chances of arbitrary constructions, they have recognized the well-settled interpretation of these phrases in the administration of criminal law, which has prevailed for ages.

[9]: 307  When in 1348, Sir John Gerberge of Royston was convicted of treason for falsely imprisoning William de Boletisford and taking his horse, until he paid him £90[10] (approximately equivalent to £80,000 in 2023), the barons compelled Edward III to agree to an Act of Parliament to restrict the definition of treason to definite limits.

"[11] Although some of the treason statutes enacted between 1352 and 1640 lost their importance over time, notably those of Henry VIII regarding his supremacy, 25 Edw 3 was the law in force cited by judges during the reign of Charles I.

A subsequent clause which requires that an "overt act" must also be proven has been held by judges to apply to all kinds of treason.

However, closer reading of the originally unpunctuated medieval document allowed for a broader interpretation, leading to the accusation by his supporters that Casement was "hanged by a comma".

The court decided that a comma should be read in the text, crucially widening the sense so that "in the realm or elsewhere" meant where acts were done and not just where the "King's enemies" might be.