2.3.1 Admissibility of opposition to be checked at every stage

The admissibility of the opposition must be checked ex officio in every phase of the opposition and ensuing appeal proceedings (T 289/91, OJ 1994, 649 and e.g. T 522/94, OJ 1998, 421; T 646/13, stating that this is established case law, and T 414/17). On the other hand, as highlighted in T 522/94, the requirement of admissibility must not be circumvented by overemphasising the ex officio principle and shifting the burden of establishing the case from the opponent to the opposition division.