53Furthermore, the effects of the withdrawal of an act of general application by the institution, in so far as that institution is the author of that act, differ from those arising from a finding of illegality by the Courts of the European Union: as the withdrawal of an act operates retroactively, it deprives any act adopted under it of its legal basis, including acts that have not been appealed, while the finding of illegality made by the court, as it does not have erga omnes effect, entails the illegality of the individual contested decision, but leaves the act of general application in the legal order without affecting the legality of other acts which have been adopted pursuant thereto and which were not challenged within the period for appeal (see, to that effect, judgment of 21 February 1974, Kortner and Others v Council and Others, 15/73 to 33/73, 52/73, 53/73, 57/73 to 109/73, 116/73, 117/73, 123/73, 132/73 and 135/73 to 137/73, EU:C:1974:16, paragraphs 37 and 38).