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The 'fiction of the lost modern grant' is a judicial device created to recognize that an Easement might arise by Prescription even if it could not satisfy the requirements forcommon law prescription. At common law, it has at least to be possible that the easement was in use in 1189. If it turns out, for example, that the land subject to a purported easement of access was under water at some time in the 13th century, the claim will be defeated.

To acquire an easement by prescription under the fiction of the lost modern grant, the easement must have been in continuous use for at least twenty years. It has to have been possible that the easement could have been the subject of an express grant (which is deemed to have been lost -- hence the name). The claim can be defeated by showing that the grant could not have been made. Of course, all the other requirements for an easement must be present as well.