Per Lord Keith of Kinkel in Armagas Ltd v Mundogas (The Ocean Frost)  Int.Com.L.R. 05:
Ostensible authority comes about where the principal, by words or conduct, has represented that the agent has the requisite actual authority, and the party dealing with the agent has entered into a contract with him in reliance on that representation. The principle in these circumstances is estoppel from denying that actual authority existed. In the commonly encountered case, the ostensible authority is general in character, arising when the principal has placed the agent in a position which in the outside world is generally regarded as carrying authority to enter into transactions of the kind in question. Ostensible general authority may also arise where the agent has had a course of dealing with a particular contractor and the principal has acquiesced in this course of dealing and honoured transactions arising out of it. Ostensible general authority can, however, never arise where the contractor knows that the agent’s authority is limited so as to exclude entering into transactions of the type in question, and so cannot have relied on any contrary representation by the principal: Russo-Chinese Bank v. Li Yau Sam  A.C. 174.