6. c. 21)[5] was an Act of the Parliament of the United Kingdom effective during World War II to facilitate the prosecution and execution of enemy spies, suspended afterwards, and repealed in 1968 or 1973, territory depending.

The law was passed on 23 May 1940, in the month after Nazi Germany invaded France and Winston Churchill became prime minister.

[7] In commending the Bill to the House of Commons, the Home Secretary, Sir John Anderson, explained why the law was necessary:[8] [T]he scope of Clause 1 of the Bill is substantially the same as the scope of the Treason Acts, but the Treason Acts might not be applicable to persons who are not normally resident within the King's jurisdiction; and moreover the Treason Acts are antiquated, excessively cumbrous and invested with a dignity and ceremonial that seems to us wholly inappropriate to the sort of case with which we are dealing here.

It was rushed through Parliament in two weeks, passing the Lords in a few minutes and receiving royal assent on the same day.

[12] The first British subject to be executed under the law was George Johnson Armstrong, who was hanged at HMP Wandsworth on 10 July 1941.

Two other British collaborators, John Amery and William Joyce, were executed on separate charges of treason.

In Gibraltar, two young Spanish men, Luis Lopez Cordon-Cuenca and Jose Martin Munoz, were tried under a similar treachery statute for being involved in acts of sabotage against the British.