In this article, we focus on Birss LJ's comments in Facebook v Voxer1 regarding the applicability of the Formstein defence to allegations of infringement under the doctrine of equivalents.

Voxer alleged its patent was infringed by the Facebook Live and Instagram Live functionality under both normal construction and the doctrine of equivalents. In addition to denying infringement, Facebook argued that it was entitled to rely on the Formstein defence.

The patent was found obvious and not infringed (on normal construction and on the basis of equivalents).  Despite these findings, Birss LJ went on to consider whether the so-called Formstein defence exists in UK law. The Formstein defence is taken from a German decision of the same name, and has also been considered by the Dutch Court of Appeal and in the US under the principle of "ensnarement".

The defence is effectively an extension of the "Gillette" principle, which provides that a patentee cannot validly claim something which was not new or was obvious at the priority date. The Formstein defence, however, takes the Gillette principle one step further: if a patent is not infringed under a normal interpretation but would be under the doctrine of equivalents, the scope of the patent will nevertheless be limited to a normal interpretation if the consequence of the broad interpretation under the doctrine of equivalents is that the patent would be invalid. Birss LJ acknowledged that the Gillette principle, despite being a well-established aspect of patent law, is rarely relied on outside of the sphere of Arrow declarations because the parties and public usually need to know whether a patent is valid (and not infringed) or invalid.  

He then went on to explain the tension between using a broader claim scope under the doctrine of equivalents as a way to invalidate a claim found valid under the normal interpretation:

"So imagine a case in which a claim on its normal construction is valid and not infringed, but a defendant's device is (i) found to infringe by the doctrine of equivalents but also (ii) found to be obvious over the prior art.  Is the right answer that the claim is infringed but invalid because its proper scope, taking into account equivalents, encompasses something obvious over the prior art; or is it valid but not infringed on the footing that part of the law of equivalents mandates that if these are the facts the equivalents doctrine does not expand the claim?  Either answer can be justified logically.  Indeed, if the matter was free from authority, given the way the scope of the claim is defined in the EPC itself, one might think the invalidity approach is a purer application of the letter of the law.  After all it is how equivalents worked when they were taken into account as part of purposive construction before Actavis."

As Birss LJ recognised, the principle has been considered by the UK Courts but always in an obiter context. It was first examined in some detail by HHJ Hacon in Technetix v Teleste2, who commented that, had the patent in issue been valid, the defendant in that case would have been entitled to a Formstein defence if such a defence existed in English law.

The principle came up in again in Emson v Hozelock3, where the parties, referring to Technetix, invited the judge to declare that such a defence forms part of English law. Again, though, given the patent had already been found invalid, Nugee J noted instead that it "should be left to a case where it would make a difference, and very probably to a higher court."

In the present case, Birss LJ did not need to consider Formstein either. However, he did share his view that the Formstein defence is the right approach to deal with obvious or novel equivalents alleged to infringe. On the other hand he considered that equivalents should not be taken into account when assessing validity. He considered that this provides certainty for patentees, who cannot be expect to do more than write claims which do not cover the prior art under a normal construction. It would, he said, "seem harsh" if a claim which was found to be valid under normal construction could nevertheless be invalidated by an equivalent in the prior art.


Since the Supreme Court handed down its decision in Actavis v Lilly, questions around how the doctrine of equivalents should apply to validity have come up but never been confronted head on.

For example, in one of the first decisions to apply the Actavis questions (albeit in an obiter context), Arnold J (as he was then) opined in Generics v Yeda4 that there could not as a matter of law be anticipation by equivalence. That approach was followed by Richard Meade QC (as he was then) in Fisher & Paykal v Resmed5, despite noting that the issue was not addressed in detail in Yeda, and that the points were at least arguable.

Addressing another aspect of validity, Arnold J confirmed in Conversant v Huawei6 that infringement by equivalence cannot give rise to an added matter objection, since it was an argument about the scope of protection and not disclosure. That position was confirmed by Birss J (as he then was) in IPCom v Vodafone7 and referred to again in this decision.

It is interesting to note that Birss LJ has already touched briefly on the relationship between validity and claim scope in a previous decision, Liqwd v L'Oreal8, where he commented that he could "see room for arguing that for validity purposes some account ought to be taken of the wider scope". It appears from the above that he has now moved away from that position.

Given these comments are again obiter, it will be interesting to see how the UK courts approach this issue when it is live. However, judicial comment from the majority of patents judges – Arnold LJ, Birss LJ and Meade J (as they are now) – seem to be aligning. Thus, patentees may be protected from the potential long arm of Actavis in revocation proceedings while benefiting when pursuing infringers. This approach also avoids complicating the patent prosecution process, as it will not be necessary for patent examiners to consider the scope of protection under the doctrine of equivalents when examining a patent application for novelty and inventive step.


1 [2021] EWHC 1377 (Pat)

2 [2019] EWHC 126 (IPEC)

3 [2019] EWHC 991 (Pat)

4 [2017] EWHC 2629 (Pat)

5 [2017] EWHC 2748 (Ch)

6 [2019] EWHC 1687 (Pat)

7 [2020] EWHC 132 (Pat)

8 [2018] EWHC 1394 (Pat)

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