What is opposition before the EPO?
Opposition is a post-grant, inter partes administrative procedure under the European Patent Convention (EPC) that allows any person to challenge a European patent granted by the EPO. An EPO revocation takes effect simultaneously across all states for which the patent was granted, making a central opposition more efficient than pursuing separate national nullity actions in each jurisdiction.
Who can file
Any person may file an EPO opposition; no commercial interest or any other form of standing need be shown.
The opposition window
Opposition must be filed within the opposition period fixed by the EPC, measured from the date the mention of grant is published in the European Patent Bulletin, not from the date the patent specification itself is published.
Permitted grounds: a closed list
Only the grounds listed in Article 100 EPC may be raised in opposition; no other basis for revocation is available in this procedure. There are exactly three:
| Ground | EPC article | What it challenges |
|---|---|---|
| Lack of patentability | Article 100(a) | Subject matter not novel or not inventive |
| Insufficient disclosure | Article 100(b) | Skilled person cannot carry out the invention |
| Extension beyond the application | Article 100(c) | Patent adds matter not present in the original filing |
Article 100(a) EPC covers patentability objections: novelty and inventive step. Article 100(b) EPC addresses whether the disclosure is sufficient for a skilled person to carry out the invention. Article 100(c) EPC targets subject matter that extends beyond the content of the application as originally filed; for divisional applications and applications filed under Article 61 EPC, the comparison is to the earlier application.
What cannot be raised
Lack of unity of invention, claim clarity under Article 84 EPC, and entitlement disputes cannot be raised as grounds for EPO opposition. This creates a procedural wrinkle: the proprietor is expected to amend claims during opposition, and those amended claims must satisfy Article 84's clarity requirement even though clarity is not an available opposition ground. In practice, a clarity defect is only actionable in opposition insofar as it underlies a patentability or added-matter argument.
How proceedings work
The Opposition Division
Oppositions are examined by an Opposition Division of three technically qualified examiners, at least two of whom must not have participated in the grant proceedings for the same patent; the examiner who did participate in the grant may not chair the division.
The Opposition Division is not limited to the facts, evidence, and arguments the parties present. It may raise grounds on its own initiative where there are clear reasons to believe those grounds are relevant.
The proprietor's rights
Once an opposition is filed, the patent proprietor may submit observations and propose amendments to the claims or description. Amendments may narrow the scope of protection but cannot extend it.
Oral proceedings
Oral proceedings may be convened at the EPO's initiative or at the request of any party. Those hearings are held in public, unlike examination oral proceedings, which are private. Because the proceedings are public, competitors and interested parties may attend.
Possible outcomes
Three results are possible once the Opposition Division reaches its decision:
| Outcome | What it means |
|---|---|
| Opposition rejected | Patent maintained as originally granted |
| Patent maintained in amended form | Claims narrowed; revised B2 specification published |
| Patent revoked | Patent cancelled across all designated EPC states |
If the opposition is found unsubstantiated, it is rejected and the patent is maintained as originally granted. If the patent cannot survive as granted but can be defended in a narrower form, the division maintains it with amendments and a revised B2 specification is published. If the patent cannot be maintained even in amended form, it is revoked in its entirety.
Each party normally bears its own costs, though the Opposition Division may order a different apportionment for reasons of equity.
Appeals
Any party adversely affected by the Opposition Division's decision may appeal to the EPO Board of Appeal.
Third-party options
Intervention after the window closes (Article 105 EPC)
A third party who is the subject of infringement proceedings or a declaratory judgment action involving the same patent may enter pending opposition proceedings by filing a notice of intervention; the intervenor is treated as a full opponent party and may raise any Article 100 EPC ground regardless of what the original opponent pleaded.
Written observations without becoming a party (Article 115 EPC)
Any person may file written observations on the patentability of a European patent application after its publication, but filers do not become parties and have no right to be heard.
Relationship to national and UPC proceedings
EPO opposition proceedings can run in parallel with revocation proceedings before national courts or the Unified Patent Court; national practice varies on whether domestic proceedings are stayed pending the EPO's decision. Because an EPO revocation takes effect across all states where the patent was in force, a successful revocation will typically end parallel national disputes as well.
Practical notes for practitioners
- No standing barrier. Because no interest need be shown, clients can challenge a patent without disclosing commercial motives. Challenge can be filed through an intermediary, provided that approach is not an abuse of process.
- Treat the grounds list as a filter, not a starting point. Map every weakness in the target patent to Article 100(a), (b), or (c) before recommending an opposition. A patent with claim clarity problems, a unity defect, or an ownership dispute cannot be revoked in opposition on those grounds alone.
- The division can go beyond the filed grounds. A narrow attack on one Article 100 ground does not cap what the division may examine. The division may raise further grounds on its own initiative, which can either uncover additional weaknesses that favor revocation or surface issues that benefit the proprietor.
- Amendments make this an iterative proceeding. The proprietor will typically use the amendment right to narrow claims around the opposition's attack. Budget for multiple rounds before oral proceedings and consider whether the final amended scope still serves your client's purpose.
- Public oral proceedings have strategic consequences. Because oral proceedings are public, competitors and press may attend. If the commercial rationale for the opposition is confidential, weigh whether the public nature creates disclosure risk before committing to an opposition.
- Written observations are low-cost and low-exposure. When a client wants to surface prior art without committing to a full opposition or revealing its identity, written observations place that art on the record at minimal cost and without triggering a costs award. This option also remains available after the opposition window has closed, as long as opposition proceedings are still pending.