A court’s decision is made per incuriam if it was made without reference to a relevant principle of decided or enacted law.

It is used most typically in a situation where a court’s attention has not been drawn to a binding earlier precedent. The effect of a decision being made per incuriam is to render it no longer binding. The per incuriam rule thus operates as an exception to the doctrine of stare decisis.

There is a useful discussion of the rule in Desnousse v Newham London Borough Council [2006] EWCA Civ 547; [2006] QB 831, at para 71:

“71 Young v Bristol Aeroplane Co Ltd [1944] KB 718, the starting point for any consideration of the per incuriam exception, spoke, at p 729, of a decision given in ignorance of the terms of a statute or a rule having the force of a statute. In Morelle Ltd v Wakeling [1955] 2 QB 379, 406 the principle was stated as applying to decisions given in ignorance of some inconsistent statutory provision or of some authority binding on the court concerned, so that in such cases some part of the decision or some step in the reasoning on which it is based is found, on that account, to be demonstrably wrong. In Miliangos v George Frank (Textiles) Ltd [1975] QB 487, 503 Lord Denning MR reviewed these and other cases, and pointed out that “a case is not decided per incuriam because counsel have not cited all the relevant authorities or referred to this or that rule of court or statutory provision”. In Duke v Reliance Systems Ltd [1988] QB 108 the failure of the court to have regard to a European Community Directive was held not to justify regarding the previous decision as per incuriam. Sir John Donaldson MR put the doctrine, at p 113, as applying only

‘where another division of this court has reached a decision in the absence of knowledge of a decision binding upon it or a statute, and that in either case it has to be shown that, had the court had this material, it must have reached a contrary decision’.”