Opposition search
Also called patent opposition search, opposition prior art search
Opposition search is a search for prior art and other evidence to support an opposition: a procedure, generally conducted before the patent office rather than a court, in which a party challenges the grant of a patent within a period set by law1.
At the European Patent Office (EPO), opposition comes after grant. Any person may give notice of opposition within nine months of the publication of the mention of grant in the European Patent Bulletin2, and the notice must indicate the facts and evidence presented in support of the grounds3.
What the search has to support
Opposition at the EPO may only be filed on the grounds in Article 100 EPC: the subject-matter is not patentable under Articles 52 to 57, the patent does not disclose the invention clearly and completely enough for a skilled person to carry it out, or the subject-matter extends beyond the application as filed5.
Lack of novelty and lack of inventive step are separate grounds within the first of these7. The same patentability requirements apply in opposition as in examination, but the EPO notes that opposition is more often based on prior art made available by oral description, by use or in any other way, not only by written description6.
| Evidence the search finds | What the notice of opposition must give |
|---|---|
| Patent documents and other publications | An indication of the facts and evidence supporting each ground relied on3. The EPO recommends filing a copy of written evidence as soon as possible, ideally with the notice4. |
| Prior use or oral disclosure before the filing or priority date | The date of the use (when), what was used (what), and the circumstances: where, how and by whom4 |
Why the nine-month period drives the search
A European opposition must be filed within nine months of the publication of the mention of grant, and the notice is not deemed filed until the opposition fee has been paid2.
The opposition is admissible only if at least one ground is substantiated with facts, evidence and arguments. Where the facts and evidence are absent or too vague to understand the case, the opposition is a mere allegation and is not admissible4.
The EPO may disregard facts or evidence not submitted in due time. In deciding whether to admit them, it considers their relevance, the stage of the proceedings and the reasons for the delay; late material that is prima facie relevant has to be taken into consideration7. This is not a substitute for a complete search within the nine months: the EPO recommends filing written evidence as soon as possible, ideally with the notice of opposition4.
From search results to a notice of opposition
- Decide the extent of the oppositionThe notice must make clear whether it is directed against all claims or only some of them4.
- Search the state of the artLook for written disclosures and for disclosures by oral description, by use or in any other way, which are more common in opposition than in examination6.
- Document any prior useRecord when the use took place, what was used, and where, how and by whom4.
- File a reasoned statementThe notice is a written reasoned statement that gives the grounds and indicates the facts and evidence supporting them3.
What the opposition decides
If the opposition is admissible, the Opposition Division examines whether at least one ground prejudices the maintenance of the patent. If so, it revokes the patent; otherwise it rejects the opposition. If the proprietor amends the patent and the amended patent meets the requirements of the EPC, the patent is maintained as amended, provided the conditions in the Implementing Regulations are fulfilled (Article 101 EPC)8.
Opponents and the patent proprietor are parties to the opposition proceedings2.
Post-grant challenges at the EPO, USPTO and CNIPA
Opposition systems differ by country. An opposition may be filed shortly before grant (pre-grant) or after grant (post-grant), and India, for example, provides both1. The tabs below show the main post-grant routes at the EPO, the USPTO and CNIPA; the list is not exhaustive.
Europe
United States
- A third party may petition for post-grant review on or before the date 9 months after grant, on any ground that could be raised under 35 U.S.C. 282(b)(2) or (3). The procedure generally applies to patents from applications subject to the AIA first-inventor-to-file provisions10.
- For first-inventor-to-file patents, an inter partes review petition may be filed after the later of 9 months from grant or the end of an instituted post-grant review. It is limited to grounds under 35 U.S.C. 102 or 103, based only on patents or printed publications11. The USPTO proposed changes to its rules for instituting inter partes review in a notice of proposed rulemaking published on 17 October 2025, and no final rule had been published in the Federal Register as of 27 October 202616.
China
- Beginning from the announcement of grant, any entity or individual who considers that the grant does not conform to the Patent Law may request the patent administration department under the State Council to declare the patent invalid (Patent Law of the PRC, Article 45)12.
- A party who does not accept the decision may file a lawsuit in the people's court within three months of receiving notice of it (Article 46)12.
Opposition search vs validity and patentability search
| Search | Question it answers | When it is done |
|---|---|---|
| Opposition search | Does at least one ground for opposition prejudice the maintenance of the patent?8 | At the EPO, within nine months of the mention of grant2 |
| Validity | Was a granted patent properly granted?9 | After grant: for litigation, licensing negotiations or due diligence9 |
| Patentability | Is the invention new and non-obvious?9 | While preparing and before filing a patent application9 |
Questions people ask
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Run Novelty SearchWho can file an opposition against a European patent?
Any person may give notice of opposition to the EPO within nine months of the publication of the mention of grant. Opponents and the patent proprietor are then parties to the proceedings2.
Can evidence of prior use support an opposition?
Yes. The state of the art relied on in opposition can include disclosures by use or oral description6. The notice must indicate when the use occurred, what was used and the circumstances of the use4.
What if relevant prior art is found after the nine months?
The EPO may disregard facts or evidence not submitted in due time. It weighs relevance, the stage of the proceedings and the reasons for the delay, and must consider late material that is prima facie relevant7. This is not a substitute for a complete search within the nine months: the EPO recommends filing written evidence as soon as possible, ideally with the notice of opposition4. Others have routes too. A third party that proves that infringement proceedings have been instituted against it, or that it has instituted proceedings for a ruling that it is not infringing after the proprietor asked it to cease the alleged infringement, may intervene in the opposition after the opposition period has expired (Article 105(1) EPC)13. Any third party may present observations on patentability without becoming a party to the proceedings (Article 115 EPC)14. After the opposition, the patent can still be revoked in national proceedings (Article 138 EPC)17 or, for patents within its jurisdiction, through a central revocation action at the Unified Patent Court15.
Does a successful EPO opposition affect every country?
The opposition applies to the European patent in all the contracting states in which that patent has effect (Article 99(2) EPC)2.
How it connects
Go from the term to the task
Read how the method works, run it on your own product or idea, then try a free tool.
- 01 / How-toHow to run a freedom-to-operate analysisScope, search, claim review and the report counsel will ask for.Read
- 02 / AI agentFTO Search AI AgentAn AI agent that finds in-force patents that could read on your product, with a claim chart for each one.Run it
- 03 / Free toolAI prior art searchDescribe an idea, get the closest patents.Try tool