Submarine patent is an informal term for a patent first published and granted long after the initial application was filed. In analogy to a submarine, its presence is unknown to the public; it stays under water, i.e., unpublished, for long periods, then emerges, i.e., granted and published, and surprises the relevant market. This practice was possible previously under the United States patent law, but is no longer practical since the U.S. signed the TRIPS agreement of the WTO: since 1995, patent terms (20 years in the U.S.) are measured from the original filing or priority date, and not the date of issuance. A few potential submarine patents may result from pre-1995 filings that have yet to be granted and may remain unpublished until issuance. Submarine patents are considered by some, including the US Federal Courts, as a procedural laches (a delay in enforcing one's rights, which may cause the rights to be lost).
Under Article 93 EPC, a European patent application shall be published as soon as possible after the expiry of eighteen months after the date of filing or, if priority has been claimed, as from the date of priority. Consequently, the key submarine patent strategy of delaying publication of a patent application for several years is not possible.
Also, it is not admissible to amend the claims of a European patent application to include subject matter that extends beyond the content of the application as originally filed. In Europe, this requirement is interpreted strictly (to protect third parties' interests ) to mean that information that is not directly and unambiguously derivable from the application as originally filed may not be added subsequently. This reduces the possibility of a granted patent having significantly different or broader scope than the patent application as published. Claim broadening, in particular, is subject to particularly careful examination under European practice.
Another limitation on amending patent applications following publication is that amended claims may not relate to unsearched subject-matter which does not combine with the originally claimed invention or group of inventions to form a single general inventive concept. Only the claims as originally filed in a European patent application are subject to a search and this rule further reduces the possibility of a European patent having significantly different or broader scope than the patent application as published. An exception to this rule is that new claims may be filed on bringing an international (PCT) application into the European phase  which may be the subject of a new search.
Divisional applications may be filed under Article 76 EPC at any time while an earlier European patent application is pending. Divisional applications are still published as soon as possible after 18 months from the priority or filing date of the parent application to make the public aware of the contents of the divisional as early as possible.
The claims of a divisional application may differ from the claims of the parent application as originally filed, potentially causing an unexpected monopoly to be claimed, so that, as the Enlarged Board of Appeal put it,
Nevertheless, Article 76(1) EPC requires that a divisional application may only be filed in respect of subject matter which does not extend beyond the content of the parent application as originally filed. The failure to meet this requirement may be remedied by amendment after filing, as confirmed by Enlarged Board of Appeal decisions G 1/05 and G 1/06 of June 2007. This language mirrors Article 123(2) EPC and therefore offers the same protection to third parties that a divisional application may not claim subject matter which was not directly and unambiguously derivable from the parent application as originally filed. In this respect, European law provides no equivalent to the controversial continuation-in-part practice of the USPTO.
In the past, when the life of a U.S. patent was 17 years from the date it was granted, submarine patents could issue decades after the initial filing date. Therefore, an applicant for a U.S. patent could benefit by delaying the issuance, and thus expiration date, of a patent through the simple, but relatively costly expedient of filing a succession of continuation applications. Some submarine patents emerged as much as 40 years after the date of filing of the corresponding application. During the extended prosecution period the claims of the patent could be tweaked to more closely match whatever technology or products had become the industry standard.
Currently, the majority of U.S. patent applications are published within 18 months after the filing date. However, the applicant can explicitly certify that they do not intend to file a corresponding patent outside the U.S. at the time they file the patent, and keep the application secret. The applicant can change their mind within the first year, but the application is then published. For continuation applications which claim priority to a previously filed application, the publication is six months after the new filing date. The changes to U.S. patent law that introduced publication at 18 months also changed the duration of the patent to 20 years from the filing date of the earliest patent application in any chain of continuation patent applications. As a result there is little benefit in postponing the grant of the patent. The enforceable life of the patent can no longer be shifted into the period when a technology has become more widely adopted, and the patent applicant must give up the chance of foreign patent protection if he is to maintain patent secrecy beyond the 18 month period. In a recent report the National Academy of Sciences has recommended that "in all cases, applications should be published during patent examinations".
Jerome H. Lemelson filed many applications that became submarine patents. He and his heirs have collected over 1.3 billion dollars (U.S.) in royalties. However, in 2004 Lemelson's estate was defeated in a notable court case involving Symbol Technologies and Cognex Corporation, which sought (and received) a ruling that 76 claims under Lemelson's machine vision patents were unenforceable. The plaintiff companies, with the support of dozens of industry supporters spent millions on this landmark case. The ruling was upheld on September 9, 2005 by a three judge panel of the U.S. Court of Appeals for the Federal Circuit under the doctrine of laches, citing "unreasonably long … delays in prosecution." Lemelson's estate appealed for a review by the full circuit en banc. On November 16, 2005, the full court declined to review the case, but, citing "prejudice to the public as a whole," extended the original unenforceability ruling to all claims under the patents in question.
The patents at issue are: U.S. Patent 4,338,626, U.S. Patent 4,511,918, U.S. Patent 4,969,038, U.S. Patent 4,979,029, U.S. Patent 4,984,073, U.S. Patent 5,023,714, U.S. Patent 5,067,012, U.S. Patent 5,119,190, U.S. Patent 5,119,205, U.S. Patent 5,128,753, U.S. Patent 5,144,421, U.S. Patent 5,249,045, U.S. Patent 5,283,641, U.S. Patent 5,351,078.