… the purpose of NOR is twofold: to inform the charterer that the vessel has completed the preliminary or carrying voyage and is at his disposal for the cargo operation; and to start the running of laytime. However to tender NOR is not sufficient for the vessel to be considered by the charterers to be at their disposal.
Per Lord Denning, MR in Compania de Naviera Nedelka Sa v Tradax Internactional Sa (The Tres Flores),  3 All ER 967 at p.970:
One thing is clear. In order for a notice of readiness to be good, the vessel must be ready at the time that the notice is given, and not at a time in the future. Readiness is a preliminary existing fact which must exist before you can give a notice of readiness; see per Atkin LJ in Akt Nordiska Lloyd v C Brownlie & Co (Hull) Ltd ((1925) 30 Com Cas 307 at 315).
In order to be a good notice of readiness, the master must be in a position to say "I am ready at the moment you want me, whenever that may be, and any necessary preliminaries on my part to the loading will not be such as to delay you." Applying this test it is apparent that notice of readiness can be given even though there are some further preliminaries to be done, or routine matters to be carried on, or formalities observed. If those things are not such as to give any reason to suppose that they will cause any delay, and it is apparent that the ship will be ready when the appropriate time arrives, then notice of readiness can be given.
Roskill LJ at p.974:
… it has long been accepted in this branch of the law that a vessel which presents herself at a loading port must be in a position to give the charterer unrestricted access to all her cargo spaces before she can give a valid notice of readiness. This state of readiness must be unqualified. It is not open to the shipowner to say: ‘Here is my ship; she is not quite ready yet but I confidently expect to be able to make her ready by such time as I consider it likely that you will in fact need her.’ The charterer has contracted for the exclusive and unrestricted use of the whole of the vessel’s available cargo space, and he is entitled to expect that that space will be placed at his disposal before he can be called on the accept the vessel as having arrived and thereafter being at his risk and expense as regards time.
By Mustill LJ in Transgrain Shipping BV v Global Transporte Oceanico SA (The Mexico I),  1 Lloyd’s Rep 507, at p.513:
At common law no notice of readiness is required at the discharging port to place the charterer under the obligation to take delivery of the cargo: he is expected to be on the lookout for the ship and for his cargo.
… the contract provides for Laytime to be started by the notice (which means a valid notice) and in no other way.
TA Shipping Ltd v Comet Shipping Ltd, (The Agamemnon),  1 Lloyd’s Rep 675, per Thomas J at p.680:
A notice of readiness which is effective to start Laytime running can only be given when the conditions set out in the charterparty for its giving have been met. A notice that does not meet those conditions is not a valid notice. Although it is often correct, when considering the statement in the notice of readiness as to the actual readiness of the vessel to discharge cargo, to speak of the truth or accuracy of a statement as to readiness, that is a way of saying that that contractual condition has been met. Again when considering the condition as to the geographical position the vessel has reached, the statement in the notice of readiness may or may not be true as to the geographical position of the vessel or whether she has reached the point that is nearest to the port or berth (as the case may be); what matters, however, is whether the condition in the charter-party for the giving of notice has been met and the vessel is, at the time the notice is given, at the point stipulated in the charter-party where notice can be given. That is the test for validity.
Per Mr Justice Moore-Bick in Glencore Grain Ltd. v Goldbeam Shipping Inc.  EWHC 27 (Comm), para.18:
It has long been recognised that the completion of the carrying voyage is a critical stage in the adventure, not least because it marks the point at which the charterers' obligation to co-operate with the owner in discharging the goods begins. For that reason it is usually also the point at which notice of readiness can be given in order to bring into operation the Laytime and demurrage provisions of the charter and at which the risk of delay to the vessel passes from the owner to the charterer. The purpose of a notice of readiness in this context is twofold: to inform the charterer that the vessel has completed the carrying voyage and is at his disposal for the discharging of cargo; and to start the running of Laytime. Unless the parties have agreed otherwise, therefore, a valid notice of readiness cannot be given until the vessel has reached her agreed destination, whether that be the port, the berth or some other place identified in the charter: see T.A. Shipping Ltd v Comet Shipping Ltd (The Agamemnon),  1 Lloyd’s Rep. 675 … However, it is open to the parties to agree that notice of readiness may be given before the vessel reaches her destination, as was done in the present case by the inclusion in clauses 6 and 37 [WIBON provision] of provisions enabling the vessel under certain circumstances to give notice of readiness before she reached the berth. The distinction between being an arrived ship and not being an arrived ship may in some cases seem rather technical, but the parties to a contract of this kind are well aware of the significance of reaching the place at which notice of readiness can be given and the consequences which follow from it. The Laytime provisions of the charter, including the provisions regulating the giving of notice of readiness, are part of a carefully worked out commercial arrangement freely entered into by both parties…