In its voluminous decision G3/14, the EPO’s Enlarged Board of Appeal (EBA) has decided to what extent clarity of the claims may be examined during opposition proceedings.
A sequence of questions was posed to the EBA relating to the extent that the clarity of claims may be examined during opposition following a specific sort of amendment (combining a dependent claim with an independent claim), but the EBA took it on itself to make sure that its answer would apply broadly to any type of claim amendment. In summary, the EBA concluded that the claims of a European patent may be examined during opposition for compliance with the clarity requirements set out in Article 84 EPC “only when, and then only to the extent that, the amendment introduces non-compliance with Article 84 EPC”.
This development of the law will be extremely welcome news for patentees, as many types of clarity attacks that have been successful for opponents in the past will no longer be available, and patentees can be more confident that making claim amendments to overcome objections will not necessarily open the flood gates for clarity attacks, as has been the case in the past. 
It appears that, to make such an attack during opposition, an opponent must now show that an amendment made by a patentee has itself introduced a lack of clarity, rather than merely caused a previously existing, but unrealised, lack of clarity to be “revealed”. Even then, the examination of clarity must be limited specifically to the allegedly unclear aspect and not extended to unrelated aspects of the claims. 
Ironically, despite the EBA going to great lengths in the decision to categorise and broadly define the various clarity attacks that might be made by opponents, the decision was delivered with a level of generality that means there is still some uncertainty as to the sort of attacks that are to be “fair game” during opposition. Indeed, the extent to which an amendment “introduces” non‑compliance with Article 84 EPC is still a point that can be expected to provide room for maneuver for opponents. There can therefore be no doubt that shrewd European patent practitioners will continue to test the boundaries of the decision in an attempt to have patents as amended during opposition revoked under a lack of clarity. It is, therefore, likely that a healthy body of EPO Board of Appeal case law interpreting this decision will follow it. 
The legal background

The opposition procedure at the EPO is exploited by a great number of parties as a relatively cost-effective way to attack granted European patents centrally at the EPO. After the EPO opposition period expires (nine months after the grant of the patent), the bundle of national patents that results from the European grant procedure must be attacked separately before each respective national office, which can be complex and costly compared to central opposition.
There are, however, only a limited number of grounds for objection available by which third parties may oppose the grant of a European patent, and an alleged lack of clarity in the claims is not amongst these grounds. Nonetheless, many European patents have been revoked in opposition proceedings due to a lack of clarity in the claims following amendment of the claims during opposition. This is because although an objection of lack of clarity is not available as a ground for opposition against granted European claims, this attack is available against claims as amended once opposition has begun by virtue of Article 101(3)(b) EPC and accompanying EPO case law (particularly EBA decisions G9/91 and G10/91 and referring decisions), which can often leave patentees with substantial uncertainty going into opposition proceedings.
Whilst this disparate treatment of claims as granted versus claims as amended in opposition appears to set an unfair double standard, the underlying reasoning behind this approach is understandable – granted patents should not be susceptible to trivial, vexatious attacks which debate the clarity of the claims, but new claim amendments made for the first time during opposition should comply with the requirements of the EPC to give the patentee and third parties due certainty. The present decision therefore sets an important standard for future EPO oppositions that will have direct and positive practical consequences for patentees going forward.