The Opinion of the Enlarged Board of Appeal on 31 May 2001 in the case of G2/98 has important implications for all patentees who might wish to make future European patent applications claiming priority from previous applications. It is also of great significance to third parties affected by such priority rights. From now on, it will be essential that patentees ensure that applications from which priority might later be sought disclose the entire subject matter upon which any future European patent application will be based. To the extent that such subject matter is not disclosed, priority will be unavailable for claims comprising it.

To understand the reasons for the Enlarged Board's decision, an overview of the relevant provisions of the European Patent Convention ("EPC") is necessary. Article 87(1) EPC provides that "an application for a patent ... shall enjoy, for the purpose of filing a European patent application in respect of the same invention, a right of priority during a period of twelve months from the date of filing of the first application". Article 87(1) therefore allows an applicant to claim priority for a European patent application from an earlier application provided this is made within a period of one year.

The meaning of same invention in Article 87(1) EPC, however, has been the subject of conflicting case law. The decisions follow two basic strands of thought. The first, so called "traditional", approach is narrow, whilst the second is somewhat more liberal in application. The question of which is the better interpretation of the law was the subject of G2/98 when it was referred by the President of the European Patent Office ("EPO") to its Enlarged Board of Appeal under the provisions of Article 112(1)(b) EPC.

The traditional approach is based on the view that the right to claim priority from a previous application is based on the extent to which subject matter claimed in the later application has been, at least implicitly, disclosed in the previous application. Thus, where the claims of a European patent application contain additional secondary features which have not been disclosed in the priority application, or where features contained in the priority application have been omitted from the later European patent application, priority may still be legitimately claimed provided that the subject matter of the European application was disclosed in the priority application. It has always been understood however that if essential features of the invention were missing in the priority application, or present in the priority application but omitted in the European application, that priority could not be claimed.

The basis for this approach is derived from the understanding that the right to obtain patent protection under the EPC, and the extent of such protection, is fundamentally linked to the disclosure made in the application as filed on a particular date. First, Article 60(2) EPC provides that the right to a European patent belongs to the person whose application has the earliest date of filing as opposed to the person who has invented first. Secondly, Article 54(3) EPC provides that such an application becomes part of the state of the art on the date of filing. The application as filed therefore immediately becomes novelty destroying against any later application for the same invention, even if made by the initial applicant. Thirdly, Article 123(2) EPC provides that the patent protection available to the applicant is determined by the extent of the disclosure made in the application as filed.

It therefore follows, according to the traditional approach, that to the extent Article 87(1) EPC allows an applicant to avoid the novelty destroying effect of Article 54(3) EPC by claiming priority from his earlier filing, the right to such priority should be determined and limited in the same way. In other words, the extent to which priority may be claimed mirrors the extent to which patent protection was or would have been available under the priority application. The right is therefore defined by the extent of the disclosure of the subject matter of the claim seeking priority made in the priority application.

The more liberal approach to establishing priority, on the other hand, permits a degree of divergence between the extent of disclosure in the priority document and the claims for which priority is sought. Thus where a claim contains an additional technical feature which has not been disclosed in the priority application, even implicitly, it may still be possible to claim priority from the previous application. This is only the case however when the additional technical feature in question is not related to the function and effect, and therefore the character and nature, of the invention. The absence of such a feature in the priority application is then not prejudicial to a claim for priority because the claim for which priority is sought is in respect of, and in substance, the same invention as that disclosed in the priority application.

According to the more liberal branch of case law therefore, an approach that only considers whether or not the claimed feature has been disclosed, even implicitly, in the priority application is regarded as being too narrow and literal an approach for obtaining a right to priority. In such circumstances, determination of priority has been regarded by the EPO as inconsistent with the provisions of Article 87(1) EPC.

When considering its Opinion in G2/98, the Enlarged Board first dealt with the question of whether the interpretation of same invention in Article 87(1) EPC employed in the traditional approach as being synonymous with same subject matter was consistent with the provisions of the EPC and the Paris Convention. Since the EPC is an agreement within the meaning of Article 19 Paris Convention, the provisions of the EPC which relate to claiming priority for the purpose of filing a European patent application are not intended to conflict with the basic provisions relating to priority contained in the Paris Convention. The Enlarged Board therefore looked to the provisions of Article 4 in the latter.

For example, pursuant to Article 4H of the Paris Convention, priority may not be refused where there are certain elements of the invention in a claim for which priority is sought which are missing from the priority application, provided that the application as a whole specifically discloses such elements. It follows, the Enlarged Board argued, that priority for a claim, i.e. an element of the invention, within the meaning of Article 4H Paris Convention, is to be acknowledged if the subject matter of the claim is disclosed in the priority application.

The traditional approach to Article 87(1) EPC is therefore not inconsistent with Article 4H Paris Convention. The Enlarged Board made similar observations with respect to the other provisions of the EPC and the Paris Convention and in particular with Article 4C(4). Article 4C(4) provides that a subsequent application concerning the same subject as a previous first application shall be considered the first application if, at the time of filing the subsequent application, the previous first application satisfies certain requirements. The Enlarged Board therefore concluded that there was no reason why the concept of same invention should be interpreted differently from same subject matter.

Having decided that the traditional approach to priority was legally acceptable, the Enlarged Board then considered the "liberal" case law. In so doing, it observed that this line of authorities posed serious disadvantages to both patentee and third parties in certain situations. For example, it considered the case of an applicant who files a national application for an invention comprising features A+B. Two months later, the applicant files a national application for A+B+C. Then, the applicant files an application for a European patent for A+B+C eleven months after filing the national application for A+B+C and taking advantage of its priority. Adopting the liberal approach, if the feature C is inessential in the sense that it is not related to the function and effect, and therefore the character and nature of the invention, the priority claimed cannot be acknowledged. This is because the first application within the meaning of Article 87(1) would have been the national application for A+B.

Other problems can arise from the point of view of novelty. Again, by way of illustration, the case of two different applicants for the same invention was considered. The following is a simplified scenario. The first applicant files a national application for A+B whilst the second files a national application for A+B+C. Again, taking the liberal approach above, feature C is considered to be inessential. Subsequently, the first applicant makes an application for a European filing for A+B+C claiming priority from the national filing for A+B. The first applicant's European application for A+B+C is now part of the "state of the art" against any European application the second applicant might make seeking priority from his national application for A+B+C. This is despite the fact that the second applicant was the first to disclose the invention comprising A+B+C.

The Enlarged Board also observed that the liberal approach to priority had procedural difficulties because there are no objective criteria for making a distinction between technical features that are related to the function and effect of an invention and those that are not. Such an approach is therefore likely to lead to arbitrariness. Furthermore, the question of whether of or not a particular technical feature is related to the function and effect of the claimed invention may change completely during the course of prosecution with the introduction of further prior art. This might have the consequence of putting a hitherto acknowledged right of priority in jeopardy which would be at variance with the requirement of legal certainty.

From the above analysis the Enlarged Board therefore concluded that a broad interpretation of same invention which draws a distinction between technical features which are related to the function and effect of an invention and those which are not is inappropriate and prejudicial to a proper exercise of priority rights. Rather, the narrower traditional approach to priority rights is necessary to ensure the principles of equal treatment of applicants, third parties and legal certainty.

Therefore, the Enlarged Board concluded that priority of a previous application in respect of a claim in a European patent application is only valid if the person skilled in the art can derive the subject matter of the claim directly and unambiguously, using common general knowledge in the technical field in question, from the previous application as a whole.