Abstract
In its landmark judgment in The Kingdom of Spain v Infrastructure Services Luxembourg S.À.R.L. and Republic of Zimbabwe v Border Timbers Ltd [2026] UKSC 9, the UK Supreme Court has delivered a decisive answer to a critical question in international investment law. The Court unanimously determined that a foreign state cannot invoke sovereign immunity to resist the registration of an arbitral award rendered under the 1965 ICSID Convention. This article provides an authoritative analysis of the judgment, focusing on the Court's clarification of the domestic law test for a state's submission to jurisdiction by prior written agreement under section 2(2) of the State Immunity Act 1978. It further examines the Court’s rigorous treaty interpretation of Article 54(1) of the ICSID Convention, finding it constitutes a clear and unequivocal waiver of adjudicative immunity. The decision firmly aligns UK law with a growing international consensus, significantly curtailing a sovereign state's ability to delay or avoid the recognition of binding ICSID awards in the United Kingdom.
Introduction
The enforcement of international arbitral awards against sovereign states often brings into sharp relief the inherent tension between a state's international legal obligations and the centuries-old doctrine of sovereign immunity. This tension was at the core of the conjoined appeals in The Kingdom of Spain v Infrastructure Services Luxembourg S.À.R.L. and Republic of Zimbabwe v Border Timbers Ltd, two cases that reached the highest appellate court in the United Kingdom. The central question was one of profound practical and legal significance: after an investment dispute has been fully arbitrated and an award rendered under the Convention on the Settlement of Investment Disputes between States and Nationals of Other States (the ICSID Convention), can a state escrow its commitment to comply by raising the shield of state immunity from the jurisdiction of domestic courts?
The United Kingdom gives effect to the ICSID Convention through the Arbitration (International Investment Disputes) Act 1966 (the 1966 Act), which provides a streamlined mechanism for registering an ICSID award as if it were a final judgment of the High Court. Spain and Zimbabwe, each the subject of a substantial adverse ICSID award, sought to set aside such registrations, arguing they were immune from the court’s adjudicative jurisdiction under section 1 of the State Immunity Act 1978 (SIA 1978). The Respondent investors countered that by becoming parties to the ICSID Convention itself, the states had submitted to the UK courts' jurisdiction, falling squarely within the exception for a "prior written agreement" under section 2(2) of the SIA 1978.
In a unanimous judgment delivered by Lord Lloyd-Jones and Lady Simler, the Supreme Court dismissed the appeals. The Court’s judgment does more than resolve a bilateral dispute; it clarifies the domestic test for the waiver of immunity by treaty and provides a masterclass in treaty interpretation under the Vienna Convention on the Law of Treaties 1969, fortifying the pro-enforcement regime of the ICSID Convention in English law.
Facts of the Case
The two appeals presented a common legal issue against a backdrop of distinct investment disputes.
The Kingdom of Spain’s Appeal
Spain had ratified the Energy Charter Treaty (ECT), which allows for investor-state arbitration under the ICSID framework. The respondent investors, Infrastructure Services Luxembourg S.À.R.L. and Energia Termosolar BV (together, "Infrastructure"), claimed that subsequent changes to Spain's regulatory regime for renewable energy violated the ECT's "fair and equitable treatment" standard, damaging their substantial investments. An ICSID tribunal agreed, awarding Infrastructure approximately €112 million in compensation. This award survived Spain's annulment challenge, which was rejected by an ICSID ad hoc committee in July 2021. Infrastructure subsequently had the award registered as a judgment in the English High Court on 29 June 2021, pursuant to the 1966 Act. Spain applied to set this registration aside, claiming state immunity from adjudicative jurisdiction under section 1(1) of the SIA 1978 .
The Republic of Zimbabwe’s Appeal
The dispute with Zimbabwe arose from investments in land by Border Timbers Ltd and Hangani Development Co. (Private) Ltd ("Border Timbers"), which were later expropriated without compensation. Border Timbers initiated arbitration under a bilateral investment treaty between Zimbabwe and Switzerland, which provided for ICSID arbitration. An ICSID tribunal dismissed Zimbabwe’s preliminary objections to jurisdiction and rendered an award in favour of Border Timbers for over US$124 million. A subsequent application by Zimbabwe to annul the award was dismissed in November 2018. After the award remained unsatisfied, the High Court registered it on 8 October 2021. Zimbabwe, like Spain, applied to set aside the registration order, asserting immunity under section 1(1) of the SIA 1978 .
Issue
The Supreme Court crystallised the primary legal dispute into a single dispositive issue, making a definitive ruling on it. The central and ultimately determinative question was whether the Appellant States, by their ratification of the ICSID Convention, had submitted to the adjudicative jurisdiction of the English courts via a "prior written agreement" as contemplated by section 2(2) of the State Immunity Act 1978. Resolution of this question demanded the Court first articulate the precise legal test governing such submissions under domestic law and then apply that test through a rigorous interpretation of the Convention's text, particularly Article 54(1).
A second issue concerning the "arbitration exception" in section 9(1) of the SIA 1978 was argued extensively before the lower courts. However, as the Court's ruling on the primary issue was entirely dispositive of the appeals, the Supreme Court expressly declined to hear submissions on, or render any decision regarding, this alternative jurisdictional basis, deeming it "unnecessary to hear argument on issue 2 or any other issues."
Petitioners’ Arguments
The case for the Appellant States, Spain and Zimbabwe, was predicated on a restrictive view of sovereign immunity waiver in English law. As summarised in the press summary for the judgment and the case law record, they argued that state immunity is such a fundamental right that any waiver by treaty must be express, requiring the explicit use of formalistic language such as "waiver" or "submission to the jurisdiction" .
They contended that Article 54(1) of the ICSID Convention could not meet this high bar. In their interpretation, the provision merely imposed an obligation on a contracting state to set up a domestic mechanism for enforcement, primarily directed at enabling states to enforce awards against investors, not the other way around. They argued the Convention could operate without any waiver of adjudicative immunity, as a state's failure to pay an award would be a breach handled at the public international law level between states, for example via Article 64 of the ICSID Convention. They posited that consenting to other states undertaking obligations did not equate to a submission to the jurisdiction of those other states' courts .
Respondents’ Arguments
The Respondent Investors, Infrastructure and Border Timbers, argued for an interpretation rooted in the reciprocal and binding nature of the text of Article 54. They submitted that the domestic test for a "prior written agreement" does not require incantation of specific words like "waiver" or "immunity." Instead, it is satisfied when the express words of a treaty, properly interpreted, clearly and unequivocally convey an intention to submit to the jurisdiction .
Applying this to the ICSID Convention, they argued that by agreeing that an ICSID award shall be enforceable "as if it were a final judgment of a court in that State," the contracting states have consented to the final result of the adjudicative process. This express obligation, to treat an award as a final judgment, is fundamentally incompatible with a claim of immunity that would prevent the court from ever reaching that stage. They emphasised that the Convention only explicitly preserves immunity for "execution" (the seizure of assets) in Article 55, while making no such provision for the prior, distinct step of "recognition and enforcement." This silence, they argued, is telling and confirms that adjudicative immunity was waived .
Courts Reasonings
The Supreme Court’s reasoning, delivered by Lord Lloyd-Jones and Lady Simler with whom the full panel agreed, proceeded in two clear stages, decisively finding in favour of the Investors on the Issue.
1. The Test for Submission under Section 2(2) SIA 1978
The Court first clarified the test for a state's submission to the jurisdiction by prior written agreement. The Court expressly stated that "a waiver of immunity by treaty requires a clear and unequivocal expression of the state's consent to the exercise of jurisdiction" . Crucially, it held that "an expression of consent does not require explicit words such as 'waiver' or 'submission'," thus rejecting the appellants' core argument for a more rigid test. The test, the Court concluded, is "whether the words used necessarily lead to the conclusion that the state has submitted to the jurisdiction" .
2. Interpretation of Article 54(1) of the ICSID Convention
Applying this test, the Court embarked on a meticulous interpretation of the ICSID Convention in accordance with Articles 31 and 32 of the Vienna Convention on the Law of Treaties.
Ordinary Meaning:The Court found that the ordinary meaning of the words in Article 54(1), namely that “Each Contracting State shall” recognise an award as binding and enforce it “as if it were a final judgment of a court in that State,” creates “mutual and reciprocal obligations” that are “inconsistent with the preservation of adjudicative immunity.” It is an obligation to achieve a specific result: the treatment of an award as a final judgment.
Context: The Court placed great weight on the architecture of Articles 53 to 55. It drew a "sharp distinction" between "recognition and enforcement" on the one hand, and "execution" against assets on the other. It found it "significant that only immunity from execution is expressly preserved" in Article 55, highlighting the absence of any corresponding preservation of adjudicative immunity in the treaty scheme. This contextual reading was fatal to the appellants' case.
Object and Purpose: The Court's teleological analysis reinforced its textual conclusion. The primary object and purpose of the ICSID Convention is to encourage the flow of private investment by providing a shield against sovereign risk. The Court reasoned that the preservation of adjudicative immunity, as argued by the states, "would render valueless the protection afforded by article 54(1)" in making enforceable the pecuniary obligations of an award as if it were a final judgment.
Supplementary Means: The Court found that its conclusion was confirmed by the travaux préparatoiresof the ICSID Convention. The historical record showed that the drafters' concern was specifically with immunity from execution, leading to the insertion of Article 55. There was nothing in the preparatory works suggesting that state immunity was intended to block the prior stage of domestic registration and recognition. The Court also noted the "broad international consensus" from courts in jurisdictions such as Australia, New Zealand, Malaysia, and the United States, which had all reached the same conclusion on the effect of Article 54.
The Final Verdict
For these reasons, the Supreme Court unanimously dismissed the appeals of the Kingdom of Spain and the Republic of Zimbabwe. The Court held that both states, by consenting to Article 54(1) of the ICSID Convention, had submitted to the jurisdiction of the English courts by a "prior written agreement" for the purposes of section 2(2) of the State Immunity Act 1978. Consequently, they were not entitled to claim state immunity to resist the registration of the ICSID awards rendered against them .
As the Court decided the case definitively on the primary issue of jurisdiction, it did not consider it necessary to rule on the alternative ground concerning the arbitration exception under section 9(1) of the SIA 1978 . The Court of Appeal's order to remit Zimbabwe's case to the Commercial Court for the determination of its reserved non-immunity defences to enforcement was allowed to stand.
Conclusion
The Supreme Court’s judgment in Spain v Infrastructure Services and Zimbabwe v Border Timbers [2026] UKSC 9 is a landmark ruling that brings welcome certainty to a previously contested area of UK investment law. By holding that Article 54(1) of the ICSID Convention operates as a clear and unequivocal waiver of adjudicative immunity, the Court has removed a significant jurisdictional hurdle that Respondent states could use to delay the enforcement of ICSID awards.
The decision has two major legal consequences. First, it firmly aligns the United Kingdom with the international consensus on this issue, reinforcing the country's reputation as a pro-arbitration and pro-enforcement jurisdiction. Second, it clarifies the domestic test under the SIA 1978, rejecting an overly formalistic requirement for explicit language and instead championing a principled approach of treaty interpretation that prioritises the ordinary meaning, context, and object and purpose of the legal text. This ruling ensures that for states that are party to the ICSID Convention, the path from a final arbitral award to a recognised UK judgment is clear, leaving sovereign immunity to operate only at the subsequent stage of asset execution, as the Convention's drafters originally intended.
Author: Khushnuma Khan | Email: khushnuma@kkassociates.co.in
Disclaimer: This article is intended for general informational and policy discussion purposes only. It does not constitute legal advice, financial advice, or a formal interpretation of law. The views expressed are based on publicly available information, prevailing statutory provisions, and reported developments as of the date of publication. Readers are advised to seek independent professional advice before taking any action based on the contents of this article. The author assumes no liability for decisions taken in reliance upon this information.