EPO Opposition Guide for US Companies

EPO opposition is one of the most efficient ways to remove a granted European patent before it becomes a commercial problem. Within nine months of the grant date, any third party can file an opposition at the European Patent Office (EPO) against a European patent granted by the EPO. The opposition is centralised, meaning a single proceeding can revoke the patent for all of the designated member states at once. For US companies facing competitor patents in Europe, this is often the most cost-effective and decisive tool available.

The nine-month window is fixed and unforgiving. The opposition period starts on the date the patent is granted and runs for nine months. There is no grace period and no extension. If you miss the window, the only remaining route is to challenge the patent in national courts on a country-by-country basis, which is usually slower and more expensive. The first task for any US company watching a competitor's European patent is to identify the grant date and calendar the opposition deadline immediately.

Central revocation is the main strategic benefit. A successful EPO opposition can revoke the patent entirely, or it can limit the claims to a scope that no longer threatens your business. Because the EPO patent is a single bundle of national rights, a revocation or amendment takes effect in every state where the patent was validated. That means one proceeding can neutralise a threat across Germany, France, Italy, Spain, the Netherlands and the other designated states at the same time. For a US company selling across Europe, this is far more efficient than defending separate national infringement actions.

Cost-effectiveness is another reason US companies use opposition. The cost of an EPO opposition is typically a fraction of the cost of parallel national litigation in just two or three major EU jurisdictions. A single national patent invalidity action in Germany or the Netherlands can exceed the cost of an entire EPO opposition. When the patent is relevant in multiple countries, the opposition route is almost always the better economic choice, provided it is filed within the nine-month window.

The grounds for opposition are limited. Under Article 100 EPC, an opposition can be based on lack of patentability (novelty or inventive step), insufficient disclosure, or extension of subject matter beyond the content of the application as filed. The most common ground is lack of inventive step. The key is to find the right prior art and to explain why a skilled person would have arrived at the claimed invention without an inventive step. This is a technical and legal argument, not a complaint about business harm or commercial unfairness.

Preparation starts with a claim construction and prior art search. The claims must be interpreted under the EPC and the EPO's case law, which is not identical to US claim construction. The prior art must be capable of destroying novelty or inventive step. It can include patent documents, scientific papers, product manuals, public presentations and prior uses. The strongest oppositions combine documentary prior art with a clear technical explanation of why the invention was obvious. The evidentiary rules are stricter than in US proceedings, so every reference must be dated and authenticated.

The opposition procedure has three main stages. First, the notice of opposition is filed within the nine-month window, together with the grounds and supporting evidence. The patent proprietor then has four months to respond. The EPO's Opposition Division examines the case and usually issues a preliminary opinion. The final stage is oral proceedings, where the opposition is argued in person and the Opposition Division decides whether to revoke, maintain in amended form, or reject the opposition. A losing party can appeal to the Boards of Appeal.

Strategic timing matters beyond the statutory deadline. If you are considering an opposition, you should start collecting prior art well before the grant date. The earlier you prepare, the stronger the evidence you can gather. You should also consider whether the patent is a real commercial threat or a nuisance. Opposing a patent sends a clear signal and may trigger a response. If the patent proprietor has a portfolio of related patents, the opposition may be only one move in a broader portfolio dispute.

For US companies, the practical decision is often made by US patent counsel in coordination with a European Patent Attorney. US counsel can identify the commercial risk, manage the business case and coordinate discovery, but the EPO opposition must be filed and prosecuted by a European Patent Attorney. The right attorney will have opposition experience, technical depth in the relevant field, and a clear view of the EPO case law on inventive step. The [how to choose a European Patent Attorney guide](/blog/how-to-choose-a-european-patent-attorney) covers the questions to ask before you engage.

Common mistakes include waiting too long to investigate, relying on US-style prior art arguments, and underestimating the claim construction differences. Another error is filing a weak opposition just to preserve the deadline. A bad opposition can be a waste of money and can make later invalidity arguments harder. The opposition should be filed only when there is a credible case of unpatentability, supported by the right evidence and a coherent technical argument.

The key takeaway is that EPO opposition is a powerful, time-limited tool for clearing competitor patents in Europe. It is centralised, cost-effective and decisive when prepared well. For US companies, the workflow is simple: monitor competitor European grants, calendar the nine-month deadline, collect prior art, and file a well-prepared opposition with a European Patent Attorney. If the patent has already passed the nine-month window, the remaining options are national revocation actions and UPC proceedings, which require a different strategy. For a broader view of European patent enforcement, see the [European Patents service page](/european-patents), and for EPO representation and opposition services, visit Bauer IP.