[3] The Act was introduced in response to public alarm at reports of wide-scale espionage, some of them fomented by popular novels and plays that dramatized the threat, supposedly from Germany, at a time of a rapid naval expansion.

Its provisions were extensive, with heavy penalties for any reporting or sketching of military, naval or air defence installations, or the harbouring of people suspected of gathering such intelligence.

[3] The Act applied in the United Kingdom, the Isle of Man, the Channel Islands, and in overseas crown territories and colonies.

[6] The Act was passed during a febrile period of "spy fever" in the years leading up to the First World War,[7][8] with widespread anti-German sentiment in Britain provoked by the Anglo-German naval arms race and events such as the Kruger telegram[9] and the Agadir Crisis.

[8] These fuelled numerous press and literary accounts of imaginary German undercover activities, such as William Le Queux's 1909 book, Spies for the Kaiser.

[10] For example, a letter in the Morning Post in May 1907 claimed that there were 90,000 German reservists and spies in Britain, with weapons caches for them in every major city, whilst an article in the 1909 edition of The Annual Register alleged that 50,000 Mauser rifles stored in a cellar near Charing Cross were intended for the 66,000 German reservists rumoured to be in London.

[3] Section 1 of the act contained tough provisions against espionage, which were extended by a 1962 Law Lords ruling to cover other activities such as sabotage and physical interference.

he shall be guilty of felony ...The words in square brackets were inserted by section 10 of, and the First Schedule to, the Official Secrets Act 1920.

"For any purpose prejudicial to the safety or interests of the State" See Chandler v. DPP [1964] AC 763, [1962] 3 All ER 142, HL "Prohibited place", s. 1(1)(a) This expression is defined by section 3 of the Act.

[16] George Blake was sentenced to imprisonment for a term of 42 years after pleading guilty to five counts of unlawfully disclosing information contrary to section 1(1)(c).

[21] It now reads: On a prosecution under this section, it shall not be necessary to show that the accused person was guilty of any particular act tending to show a purpose prejudicial to the safety or interests of the State, and, notwithstanding that no such act is proved against him, he may be convicted if, from the circumstances of the case, or his conduct, or his known character as proved, it appears that his purpose was a purpose prejudicial to the safety or interests of the State; and if any sketch, plan, model, article, note, document, or information relating to or used in any prohibited place within the meaning of this Act, or anything in such a place [or any secret official code word or pass word], is made, obtained, [collected, recorded, published], or communicated by any person other than a person acting under lawful authority, it shall be deemed to have been made, obtained, [collected, recorded, published] or communicated for a purpose prejudicial to the safety or interests of the State unless the contrary is proved.The words in square brackets were inserted by the Official Secrets Act 1920.

Cases under this section Command papers on this section For the purposes of this Act, the expression "prohibited place" means— [(a) any work of defence, arsenal, naval or air force establishment or station, factory, dockyard, mine, minefield, camp, ship, or aircraft belonging to or occupied by or on behalf of His Majesty, or any telegraph, telephone, wireless or signal station, or office so belonging or occupied, and any place belonging to or occupied by or on behalf of His Majesty and used for the purpose of building, repairing, making, or storing any munitions of war, or any sketches, plans, models or documents relating thereto, or for the purpose of getting any metals, oil, or minerals of use in time of war]; (b) any place not belonging to His Majesty where any [munitions of war], or any [sketches, models, plans] or documents relating thereto, are being made, repaired, [gotten,] or stored under contract with, or with any person on behalf of, His Majesty, or otherwise on behalf of His Majesty; and (c) any place belonging to [or used for the purposes of] His Majesty which is for the time being declared [by order of a Secretary of State] to be a prohibited place for the purposes of this section on the ground that information with respect thereto, or damage thereto, would by useful to an enemy; and

(d) any railway, road, way, or channel, or other means of communication by land or water (including any works or structures being part thereof or connected therewith), or any place used for gas, water, or electricity works or other works for purposes of a public character, or any place where any [munitions of war], or any [sketches, models, plans] or documents relating thereto, are being made, repaired, or stored otherwise than on behalf of His Majesty, which is for the time being declared [by order of a Secretary of State] to be a prohibited place for the purposes of this section, on the ground that information with respect thereto, or the destruction or obstruction thereof, or interference therewith, would be useful to an enemy.The words in square brackets were inserted or substituted by the Official Secrets Act 1920.

This section now provides: (1) If a justice of the peace is satisfied by information on oath that there is reasonable ground for suspecting that an offence under this Act has been or is about to be committed, he may grant a search warrant authorising any constable [named therein] to enter at any time any premises or place named in the warrant, if necessary, by force, and to search the premises or place and every person found therein, and to seize any sketch, plan, model, article, note, or document, or anything of a like nature or anything which is evidence of an offence under this Act having been or being about to be committed, which he may find on the premises or place or on any such person, and with regard to or in connexion with which he has reasonable ground for suspecting that an offence under this Act has been or is about to be committed.

(2) Where it appears to a superintendent of police that the case is one of great emergency and that in the interest of the State immediate action is necessary, he may by a written order under his hand give to any constable the like authority as may be given by the warrant of a justice under this section.The words "named therein" in square brackets in section 9(1) were repealed for England and Wales by section 119(2) of, and Part I of Schedule 7 to, the Police and Criminal Evidence Act 1984.