“Plausibility” is not a ground for invalidity of a patent according to the European Patent Convention or the UK Patents Act. Nevertheless, it has become something of a buzzword in cases considered by the UK courts in recent years and it has featured in decisions of the Boards of Appeal of the EPO for some time now. As such, it is important to understand its significance and to ensure that patent applications relate to a “plausible” invention from the outset.

The origin of the concept of plausibility can be traced back to the 1995 EPO Board of Appeal decision T 939/92 (AgrEvo), which found the patent invalid for lack of an inventive step because it was not considered credible that all of the compounds claimed had herbicidal properties. The term “plausibility” was not actually used in this decision, but in the 2005 Board of Appeal decision T 1329/04 (John Hopkins), the patent application was held to contain sufficient information to make it “at least plausible” that a solution to the technical problem had really been found.

A leading textbook for UK patent practitioners, Terrell on the Law of Patents, was first published in 1884. However, it was not until the 2016 edition that the concept of plausibility was included. Its inclusion has been in response to a significant increase in the number of cases in which the issue of plausibility has been considered during UK High Court proceedings, especially in life sciences cases. Plausibility is a particular issue in the field of pharmaceuticals because even a small change to the chemical structure of a medicament or to the dosing regimen can have a significant and unpredictable effect on the medicament’s efficacy.

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