The Act was described as "purely procedural" by the House of Lords in R v Joyce,[3] but in fact extended the definition of high treason.

It was also made treason to say that the king was a heretic or usurper, in writing or (for the third offence only) in speech.

However the Act amended procedure by stating that a person accused of committing treason only by "open preaching or words" must be prosecuted within three months of the offence, instead of the 30-day limit previously established by the Treason Act 1547.

The Act also prohibited the widow of a man convicted of treason from claiming her dower.

Section 20(3) of, and the fourth schedule to, the Administration of Justice (Miscellaneous Provisions) Act 1938 (1 & 2 Geo.