Re-examining Legal Certainty: Timing, Not Values, in the UK-Singapore Debate on the Doctrine of Equivalents

By Seraphine Lai

Category Award: Intellectual Property Law, UKSLSS Essay Competition 2025

Introduction

The doctrine of equivalents has long occupied a contentious place in patent law. Its promise is intuitive: patentees should not be deprived of protection merely because an infringer has made immaterial variations to avoid the literal wording of a claim. Yet its costs are equally apparent. By extending patent protection beyond the language of the claims, the doctrine risks undermining legal certainty and the public notice function of patents.

The divergence between the United Kingdom and Singapore following the UK Supreme Court’s decision in Actavis v Eli Lilly [1] and the Singapore Court of Appeal’s rejection of that approach in Lee Tat Cheng v Maka GPS Technologies [2] has therefore often been framed as a clash between fairness to patentees and certainty for third parties [3]. This article argues that such a framing is misleading.

Instead, the UK-Singapore divide is better understood as a disagreement over when legal certainty should be demanded within the patent system. While UK law after Actavis tolerates uncertainty at the infringement stage in order to achieve equitable outcomes ex post, Singapore law insists on certainty ex ante by binding patentees to the claims they have chosen to draft. This temporal distinction reflects deeper differences in how each jurisdiction conceptualises the patent system itself: as an equitable monopoly deserving corrective protection in the UK, and as a regulatory instrument requiring clear boundaries in Singapore.

The Doctrine of Equivalents and Actavis v Eli Lilly

Prior to Actavis, UK patent law rejected any freestanding doctrine of equivalents. In Kirin-Amgen v

Hoechst Marion Roussel [4], the House of Lords held that the scope of patent protection was exhaustively determined through purposive construction of the claims, properly interpreted in light of the specification. On this view, there was no doctrinal space for extending protection beyond the language of the claims themselves. This position constituted a settled orthodoxy within UK patent jurisprudence.

The orthodoxy was displaced by the Supreme Court’s decision in Actavis. The Court adopted a two-stage approach to infringement [5]. First, the court must determine whether the alleged variant falls within the claims as a matter of normal interpretation. Where it does not, the court must then consider whether the variant nonetheless infringes because it differs from the claimed invention in ways that are immaterial. This second stage, assessed through reformulated “Protocol questions”, amounted to the formal recognition of a doctrine of equivalents within UK law.

The immediate doctrinal consequence of Actavis was to decouple claim construction from infringement [6]. Whereas purposive interpretation previously defined the outer limits of patent protection, infringement could now be found even where the claim language, properly construed, was not satisfied. This shift expanded the protective reach of patents and authorised judicial intervention where drafting failed to capture the substance of the invention.

Singapore’s Rejection of the Doctrine of Equivalents

In sharp contrast, the Singapore Court of Appeal in Lee Tat Cheng [7] expressly rejected the adoption of Actavis and the doctrine of equivalents. While acknowledging that the Singapore Patents Act was modelled on its UK counterpart, the Court emphasised that Singapore was not bound by the Protocol on the Interpretation of Article 69 of the European Patent Convention, which had significantly influenced the UK Supreme Court’s reasoning in Actavis [8].

Singapore instead reaffirmed its commitment to purposive construction as the sole determinant of patent scope. Under this approach, claims are interpreted through the eyes of the skilled person, in light of the description and drawings, to identify the essential features of the invention. Crucially, however, the scope of protection cannot extend beyond what the claims, purposively construed, encompass. Where a variant falls outside the claim language, there is no infringement, regardless of functional similarity [9].

The Court’s rationale was explicit: patentees should be bound by the language they chose to define their monopoly, and third parties should be entitled to rely on that language when designing around patents. Extending protection beyond the claims through post-hoc assessment of equivalence was therefore rejected as an improper redistribution of drafting risk [10].

Moving Beyond Fairness versus Certainty

The prevailing characterisation of Actavis and Lee Tat Cheng as reflecting a conflict between fairness and certainty is analytically incomplete. While much of the literature continues to frame the doctrine of equivalents as a trade-off between equitable protection for patentees and legal certainty for third parties, this binary obscures the more fundamental point of divergence: the institutional choice of when legal uncertainty is tolerated, and who is required to bear it.

UK law after Actavis does not abandon legal certainty as a value; rather, it accepts that certainty need not be achieved exclusively at the stage of claim construction [11]. Instead, Actavis reflects a deliberate institutional willingness to tolerate uncertainty during the life of the patent, permitting infringement risk to be resolved only ex post through judicial assessment of equivalence. The doctrine of equivalents therefore reallocates the risk of linguistic imprecision away from patentees and onto third parties, who must accept that a design-around may later be characterised as infringing. The fairness secured is corrective rather than distributive: it enables courts to remedy under-inclusive claim drafting after the fact, rather than requiring patentees alone to bear the consequences of linguistic constraint.

Singapore law adopts the opposite stance. By insisting that the scope of protection be conclusively fixed at the stage of claim construction, Singapore patent law refuses to defer certainty beyond interpretation. The burden of precision is placed squarely on the patentee at the drafting stage, and courts decline to recalibrate patent scope through ex post adjudication [12]. While purposive construction continues to provide a measure of interpretive generosity, residual uncertainty is not tolerated once the claims have been construed. The divergence therefore reflects a principled institutional choice about when uncertainty becomes unacceptable, rather than a disagreement over the value of certainty itself.

Patents as Equity vs Patents as Regulation

This divergence is best understood by reference to deeper differences in legal culture and institutional role. In the UK, the doctrine of equivalents reflects a historically entrenched equitable conception of adjudication, in which courts are understood as active participants in correcting the rigidity of formal legal instruments [13]. Patent law developed within a common law tradition that has long been comfortable with judicial mitigation of formalism, particularly where strict adherence to text would frustrate substantive justice. Against this backdrop, the willingness of UK courts in Actavis to intervene beyond the literal scope of claims is less an anomaly than a continuation of an established judicial posture.

On this view, patents are treated not merely as textual grants, but as vehicles for protecting the underlying inventive contribution. The doctrine of equivalents functions as a judicial safety valve, enabling courts to prevent the appropriation of the inventive concept through immaterial variation where drafting precision proves inadequate. That willingness to correct reflects confidence in the judiciary’s institutional competence to balance competing interests ex post.

Singapore’s rejection of the doctrine reflects a contrasting regulatory conception of patents, shaped by a legal culture that emphasises certainty, administrability, and the disciplined allocation of institutional roles. As a relatively young patent jurisdiction, Singapore has consistently framed intellectual property rights as state-granted monopolies whose legitimacy depends on clearly defined and publicly ascertainable boundaries [14]. This orientation is reinforced by a broader doctrinal tendency to hold sophisticated actors strictly to the language of instruments they have chosen to draft, particularly where third-party reliance is foreseeable.

Within this framework, extending protection beyond the claims through judicial assessment of equivalence is viewed not as equitable correction, but as regulatory distortion. It risks undermining the notice function of patents, complicating competitive planning, and shifting responsibility from patentees to courts. Singapore law therefore deliberately internalises drafting risk within the patentee’s domain, not out of hostility to innovation, but out of concern for systemic coherence and institutional restraint [15].

Accordingly, the UK-Singapore divergence should not be understood as reflecting differing commitments to fairness or innovation. Rather, it reflects competing judgments about the proper role of courts in managing the consequences of linguistic imprecision. UK law entrusts courts with correcting patent scope through adjudication; Singapore law insists that those consequences be resolved through drafting discipline before the patent enters the market. The divergence is institutional, not ideological.

Conclusion

The doctrine of equivalents remains one of the most contested features of patent law, not because its objectives are unclear, but because it forces legal systems to confront difficult questions about uncertainty, equity, and institutional competence.

By re-locating the UK-Singapore divergence as a disagreement over the timing of legal certainty, this article moves beyond the conventional fairness-versus-certainty framing. The UK’s adoption of the doctrine of equivalents reflects a willingness to tolerate uncertainty ex post in pursuit of equitable outcomes, while Singapore’s rejection of the doctrine reflects a commitment to ex ante certainty grounded in a regulatory understanding of patent monopolies.

The divergence therefore reveals not conflicting values, but different answers to a shared institutional

question: at what point should the law demand clarity in defining the scope of exclusive rights?

References

[1] Actavis UK Ltd v Eli Lilly and Co [2017] UKSC 48, [2018] 1 All ER 171

[2] Lee Tat Cheng v Maka GPS Technologies Pte Ltd [2018] 1 SLR 856 (CA)

[3] Norman Pumfrey, ‘The Doctrine of Equivalents in Various Patent Regimes: Does Anybody Have It Right?’ (2009) 11 Yale J Law & Tech 261

[4] Kirin-Amgen Inc v Hoechst Marion Roussel Ltd [2004] UKHL 46, [2005] RPC 9

[5] Actavis UK Ltd v Eli Lilly and Co [2017] UKSC 48, [2018] 1 All ER 171

[6] Darren Smyth and Azadeh Vahdat, ‘The Doctrine of Equivalents in the UK Following Actavis v Lilly’ in Hayleigh Bosher and Eleonora Rosati (eds), Developments and Directions in Intellectual Property Law (OUP 2023)

[7] Lee Tat Cheng v Maka GPS Technologies Pte Ltd [2018] 1 SLR 856 (CA)

[8] Alec van der Byl, ‘Scope of Patent Protection’ in Justine Pila and Paul Torremans (eds), European Intellectual Property Law (OUP 2019). [9] Ng-Loy Wee Loon, Law of Intellectual Property of Singapore (3rd edn, Sweet & Maxwell 2021) ch 8

[10] Bryan Ching, ‘Retaining the Catnic–Improver Approach in Patent Law: Why Singapore Should Not Adopt the Doctrine of Equivalents’ (2018) 30 Singapore Academy of Law Journal 871

[11] Crawford Jamieson, ‘In Defence of a UK Doctrine of Equivalents’ (2019) 41 EIPR 147

[12] Ng-Loy Wee Loon, Law of Intellectual Property of Singapore

[13] Bryan Ching, ‘Retaining the Catnic–Improver Approach in Patent Law: Why Singapore Should Not Adopt the Doctrine of Equivalents’ (2018) 30 Singapore Academy of Law Journal 871

[14] Norman Pumfrey, ‘The Doctrine of Equivalents in Various Patent Regimes: Does Anybody Have It Right?’ (2009) 11 Yale J Law & Tech 261

[15] Lionel Bently and Brad Sherman, Intellectual Property Law (5th edn, OUP 2018)