[1] It decided that the royal prerogative does not entitle the Crown to take possession of a subject's land or buildings for administrative purposes connected with the defence of the realm without paying compensation.

In the meantime, petition of right proceedings had previously been heard by the Court of Appeal in July 1915,[5] concerning the requisition of Shoreham Aerodrome, an airfield on the south coast, by another department, the War Office, and when the owner's appeal came to be heard by the House of Lords in July 1916, additional relevant historical information was available, resulting in an outcome that enabled the owner (The Brighton-Shoreham Aerodrome Ltd) to obtain compensation under the Defence Act 1842.

[6] In 1919, the Court of Appeal (Sir Charles Swinfen Eady MR and Warrington LJ; Duke LJ dissenting),[7] reversing the decision of Mr Justice Peterson in the High Court, decided that De Keyser's Royal Hotel Ltd, as the hotel's owner, was entitled to compensation in the manner provided by the Defence Act 1842.

In an introduction to an authoritative commentary published soon after the decision, Simon described it as one of the leading cases in constitutional law, concerned with establishing the rights of individual citizens in the face of exceptional interference by the Executive.

[11] The principles established in the De Keyser's Royal Hotel case have been referred to in later judgments when the government's claim to rely on the exercise of power under the royal prerogative has been challenged, such as Laker Airways Ltd v Department of Trade (1976), concerning the revocation of the commercial airline operator's licence of Laker Airways,[12] R v Secretary of State for the Home Department, ex parte Fire Brigades Union (1995), concerning changes to the Criminal Injuries Compensation Scheme,[13] and most recently in R (Miller) v The Prime Minister and Cherry v Advocate General for Scotland (2019), concerning the royal prerogative to prorogue Parliament.