UK Supreme Court Clarifies State Immunity in ICSID Award Enforcement

1. Introduction and Background

The recent UK Supreme Court decision in The Kingdom of Spain v Infrastructure Services Luxembourg S.À.R.L. and Border Timbers v Zimbabwe [2026] UKSC 9 marks a significant development in the enforcement of ICSID awards against sovereign states in England. This judgment follows the Court of Appeal’s pivotal ruling, previously analysed in our client alert, which clarified the interplay between Article 54(1) of the ICSID Convention and the State Immunity Act 1978 (the SIA).

The central issue was whether states could invoke sovereign immunity to resist the registration and enforcement of ICSID awards in English courts, a question of considerable importance for both investors and sovereign states engaged in investment arbitration.

2. Supreme Court Judgment – Key Holdings and Reasoning

The Supreme Court has now confirmed that by acceding to the ICSID Convention, and specifically Article 54(1), contracting states, including Spain and Zimbabwe, have given a sufficiently express and clear submission to the adjudicative jurisdiction of UK courts. This constitutes a waiver of immunity otherwise afforded by section 1(1) of the SIA for the purposes of proceedings to recognise and enforce ICSID awards. The Court emphasised that the reciprocal obligations in Article 54(1) are fundamentally inconsistent with maintaining adjudicative immunity: each contracting state is obliged to recognise and enforce ICSID awards, including as against itself, in the territories of other contracting states. This mutual and reciprocal obligation, the Court reasoned, necessarily entails a waiver of adjudicative immunity.

Importantly, the Court clarified that section 2(2) of the SIA requires an express, not implied, submission to jurisdiction, but that the contracting states’ agreement to Article 54(1) meets this threshold. The Court stated that a waiver of immunity by treaty requires a clear and unequivocal expression of consent, which does not demand explicit use of the words “waiver” or “submission” if the treaty’s language and context make the intention clear. Instead, the question is whether a proper interpretation of the treaty language necessarily entails consent to the exercise of jurisdiction. The Court’s approach aligns with the approach to treaty interpretation under the Vienna Convention on the Law of Treaties, which looks to the ordinary meaning of the treaty’s wording, in context, and in light of the treaty’s object and purpose.

The Court also distinguished between immunity from adjudicative jurisdiction (addressed in this case) and immunity from execution, which remains preserved by Article 55 of the ICSID Convention. The lack of an equivalent preservation of immunity under Article 54(1) for adjudicative jurisdiction reinforced the Court’s conclusion that contracting states submitted to the jurisdiction of contracting states’ national courts for recognition and enforcement of ICSID awards.

However, the Supreme Court (like the Court of Appeal) declined to consider arguments relating to the ‘arbitration exception’ in section 9 of the SIA (“Where a State has agreed in writing to submit a dispute which has arisen, or may arise, to arbitration, the State is not immune as respects proceedings in the courts of the United Kingdom which relate to the arbitration”), including in particular whether parties to ICSID awards are precluded by way of issue estoppel from arguing that an ICSID tribunal lacked jurisdiction. As we noted in our prior article, the Court of Appeal had indicated (obiter) that the court would need to satisfy itself that the state in question had agreed in writing to submit the dispute in question to arbitration. This issue will therefore arguably remain open in cases where section 2(2) of the SIA is not available.

3. England’s Pro-arbitration and Pro-enforcement Reputation

The Supreme Court decision therefore resolves with finality, prior uncertainty and conflicting lower court reasoning, notably departing from the view that Article 54(1) was insufficiently clear as a submission to jurisdiction. The Supreme Court’s reasoning reinforces the UK’s commitment to the ICSID Convention’s objectives and the principle of mutual enforceability among contracting states. The judgment further cements England’s reputation as a pro-arbitration and pro-enforcement jurisdiction. By confirming that sovereign immunity cannot be invoked to resist the registration and enforcement of ICSID awards, the decision provides legal certainty for investors and aligns England with other leading jurisdictions, such as the United States and Australia.

4. But . . . Execution Immunity Remains a Live Issue

While Article 54(1) of the ICSID Convention operates as a waiver of adjudicative immunity, Article 55 expressly preserves immunity from execution measures. The Supreme Court’s decision and the travaux préparatoires for the ICSID Convention confirm that nothing in Article 54 derogates from the law in force in any contracting state relating to immunity from execution. As a result, even after an ICSID award is recognised and registered as a judgment, the award creditor must still overcome the hurdle of identifying state assets that are not immune from execution under domestic law. This distinction between adjudicative and execution immunity is recognised across jurisdictions, and the practical effect is that immunity from execution remains a live issue, subject to further litigation and statutory exceptions.

Of course, Article 55 does not confer absolute immunity from execution; it simply preserves whatever immunity exists under domestic law. So, for example, although state property is generally immune from execution, section 13(4) of the SIA permits execution against property in use or intended for use for commercial purposes. Accordingly, where a state owns assets in the UK that are genuinely commercial in character, execution may be possible, notwithstanding Article 55. The availability of such execution turns on the nature and use of the particular assets, rather than on the ICSID Convention itself.

Further, it remains open to a foreign investor to plead issue estoppel on the basis of the registered award, insofar as the state attempts to re-litigate in court issues that have already been decided in the award, as that does not require the award itself to be executed against the state.

Ultimately, registration of an ICSID award secures its status as a judgment, but the separate and fact-sensitive question of execution remains governed by domestic immunity rules and the particular assets against which execution is sought.

5. Watch This Space

The Supreme Court noted that Zimbabwe had reserved its right to advance non-immunity defences to enforcement of the award. These defences have not yet been adjudicated and have been remitted to the Commercial Court for further determination. The Court expressly stated that it expressed no view on the merits of these objections, which may include exceptional matters recognised in prior case law as remaining open by reason of the award being treated like any other final judgment.

6. Concluding Remarks

The UK Supreme Court’s judgment provides authoritative clarity that Article 54(1) of the ICSID Convention operates as a waiver of sovereign immunity and a submission to English court jurisdiction for the recognition and enforcement of ICSID awards. This aligns England with international consensus, strengthens its pro-arbitration stance, and assures investors of robust enforcement mechanisms. However, immunity from execution remains preserved under Article 55, and it remains open to a state to raise non-immunity defences. Award creditors should be mindful that, while registration and recognition of ICSID awards in England are now secure from immunity challenges, the practical execution of an award against state assets may still face significant hurdles. The litigation landscape will continue to evolve as courts address the scope and application of execution immunity in future cases.

It remains to be seen whether the Supreme Court’s reasoning will have any bearing on the appeal of the Commercial Court’s judgment in relation to state immunity and the New York Convention in CC/Devas et al. v The Republic of India [2025] EWHC 964 (Comm) (which we reported in our client alert).

We are grateful to our trainee solicitor, Emma Masters-Oca, for her contributions to this client alert.