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Toxic Priority and Poisonous Divisionals – An Antidote on the Horizon?

During the appeal filed in relation to European patent EP 0 921 183, the Board of Appeal decided to refer a number of questions to the Enlarged Board of Appeal (EBoA), in order to determine the extent to which partial priority can be claimed, and what effect this has on priority applications becoming prior art (so called “toxic priority”).

There are currently conflicting decisions on partial priority, and as a result, some legal uncertainty. Patent proprietors and legal professionals eagerly await clarification from the EBoA, as the validity of many patents may hang on this decision.


Under Art. 88 EPC, it is possible for a European patent application to claim priority from multiple earlier documents, and in particular for multiple priorities to be claimed for a single claim.  For example, a single claim which covers two alternative and separable features A and B, may claim priority from one document for feature A and another document for feature B.  The claim will have different priority dates for the different alternatives in the claim.

However, “toxic priority” occurs when a priority document becomes novelty-destroying prior art for a subsequent priority-claiming application.  For example, if a European application (EP1) discloses a specific species (copper) and the subsequent priority-claiming European application (EP2) discloses the broader genus (any metal), EP2 is not fully entitled to priority.   Thus, EP1 forms prior art against EP2.

Similarly, “poisonous divisionals” arise when a divisional application becomes novelty-destroying prior art for its own parent application.  For example, if a divisional application (EP3) is entitled to priority from EP1, but the parent application EP2 is not, then the divisional EP3 will have an earlier effective priority date than the parent application.  This leads to the odd situation where divisional EP3 can form novelty-destroying prior art against the parent EP2.

During opposition of EP 0 921 183, the EPO held that the patent claims lacked novelty over its own priority application, because the granted claim was a generalisation of a specific disclosure in the priority application.  This is a case of “toxic priority”.  During appeal, the Board of Appeal considered how the novelty of the claims could depend on whether a patent could validly claim a right of priority when the claims are broader (more generic) than the disclosure in the priority application.

G2/98 stated that the “use of a generic term or formula in a claim for which multiple priorities are claimed… is perfectly acceptable… provided that it gives rise to the claiming of a limited number of clearly defined alternative subject-matters.  However, this condition is not explicitly defined, and consequently, G2/98 has been interpreted in subsequent decisions in two different ways. In EP 0 921 183, the conclusion reached by the opposition division is in line with some earlier decisions (e.g. T1877/08 and T1496/11), but is in conflict with other decisions (e.g. T1222/11).  The Board of Appeal has asked the EBoA to clarify the condition.

What Does This Mean?

No problems arise if a European patent application is identical to the priority application. However, a priority application often relates to a specific initial implementation, while the subsequent priority-claiming European application filed 12 months later often contains additional implementations and broader claims.  In this case, it is useful to retain all of the disclosure from the priority application in the later application, including all explicitly-recited ranges and sub-ranges (or generic and specific features).  This at least allows the possibility of later amendment to priority-entitled subject matter.

It may be some time before the EPO’s Enlarged Board reaches a conclusion.  In the meantime, it is important to take care when drafting the initial (priority) application to cover any potential future, foreseeable developments of the invention.  This of course may mean that the cost of filing an initial application is high, but this may be outweighed by the risk of losing a priority claim.

Please contact us at mail@tlip.co.uk for confidential advice if you believe your patent portfolio may be affected by the issues discussed above.