The inconsistency in the EPO Boards of Appeal in the problem of assessing partial priority has been recognised and is being referred to the EPO Enlarged Board of Appeal.

When going through the process of trying to get a patent for an invention, it is quite common for a first application to be filed that defines the invention in narrow terms, after which a second application is filed, claiming priority to the first, describing and claiming the invention more broadly. Under European law, this would not normally result in any problems, provided there is no intervening disclosure of the invention outside the narrow scope of the first application. Some examples where this would be the case are: i) the first application describing a specific chemical compound, while the second claims a broader class of compounds; ii) the first application describing a narrow range of parameters, which is broadened in the second application; and iii) the first application describing a specific use for a method, while the second describes and claims the same method for a wider class of uses.

If the invention is defined in the second application in a single claim, a question that can sometimes arise is how to determine the priority date of the claim. If the claim is entitled to the earlier priority date, does this apply for the whole scope of the claim, or only part of it? With no intervening disclosure this should not be an issue because the priority date would not affect the second application’s validity. Problems can, however, arise when the first application is also a European patent application that is published independently of the second application. The first application then becomes novelty only prior art for the second application, but only to the extent that the second application does not validly claim priority. If the claimed invention in the second application cannot validly claim priority, one interpretation is that the first application prevents the second application from claiming the broader scope. One solution to this problem would be to limit the claimed invention to the narrow scope of the first application, or to disclaim the narrower scope. This may, however, be either undesirable or not possible in many cases.

An alternative solution to the problem is to show that the part of the invention defined in the second application that does not have a valid claim to priority is inevitably novel over the first application, while the part that does have a valid claim to priority is not affected. This is demonstrated graphically in the accompanying diagram. The claim scope of the second application entirely encompasses the disclosure of the first application (the priority document). The part that covers the priority document disclosure has a valid claim to priority, so the first application is not prior art, while the rest of the claim scope has a later priority but is inevitably novel. The combined effect is that the first application cannot affect the second if it also publishes as a European application.

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The EPO Boards of Appeal have not been consistent in how the problem can be addressed. In several decisions (T 1877/08, T 476/09, T 1443/05 and T 1127/00) the Board decided that partial priority could not be validly claimed when broadening a range or a chemical formula. In two more recent decisions (T 1222/11 and T 571/10), however, the alternative solution has been approved. This inconsistency in how to resolve the issue has now been recognised in a recent appeal case regarding a similar issue (relating to European Patent Application No. 98203458.9), where the Board have decided to refer the issue to the EPO Enlarged Board of Appeal. The exact question to be asked has not yet been determined, but will in general terms relate to whether, and to what extent, partial priority can be validly claimed in a later application that claims a broader scope compared to a disclosure in a priority application. How this is resolved will have a major effect on how priority is claimed in European applications.