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Author: Smriti Kalra
The UK Supreme Court's recent judgment in The Kingdom of Spain v Infrastructure Services [2026] UKSC 9 is a significant decision consolidating recent trends in judicial opinion on the relationship between state immunity legislation and ICSID awards. We briefly survey the ruling and its consequences.
This appeal concerned two cases, heard together, in which investors sought registration of ICSID awards in the UK. The first, Infrastructure Services, concerned an Energy Charter Treaty award against Spain in favour of investors from two other EU Member States. The second, Border Timbers, involved an award against Zimbabwe made under the Switzerland/Zimbabwe BIT.
In both sets of proceedings, the States resisted registration of the award on the basis of immunity from the English court’s adjudicative jurisdiction under the State Immunity Act 1978 (SIA). An ICSID award, once registered in England, is treated as a final judgment of the English High Court — so registration is a necessary first step towards enforcement. At first instance, these arguments failed, albeit for different reasons (see here and here). Subsequently, these were also rejected by the Court of Appeal, which held that registration of an ICSID award engages the adjudicative jurisdiction of the English courts, such that analysing the SIA is relevant to the process. However, it also held that Article 54 of the ICSID Convention constituted a prior written agreement to that jurisdiction by the States party to it (Contracting States), which, pursuant to s.2 SIA (in particular s.2(2)), displaced their immunity to that registration process.
Before the Court of Appeal, a second potential basis for the States not having immunity in respect of the ICSID registration proceedings was s.9 SIA. This provides that a state has no adjudicative immunity in UK court proceedings relating to an arbitration where the state agreed in writing to the arbitration. Insofar as the investors needed to rely on s.9 SIA, the States argued that they were then entitled to challenge the respective ICSID tribunals’ jurisdiction afresh (in Spain’s case, this was based on intra-EU objections arising from Achmea and Komstroy; in Zimbabwe’s case, it concerned the scope of the relevant arbitration provision in the BIT). Given the Court of Appeal’s conclusion on s.2 SIA/Article 54, it did not have to rule on this. But it indicated, obiter, an inclination to accept that s.9 (if applicable) required the court to satisfy itself that the relevant state had agreed to arbitration.
Before the UKSC, the main issue that it heard argument on and decided was the key question of whether Article 54 ICSID Convention is a written submission to the court’s adjudicative jurisdiction to register an ICSID award for the purposes of s.2(2) SIA?
As indicated above, if so, then under s.2 SIA, the States would not be entitled to immunity from the proceedings to register the awards (s.17(2) SIA confirms that such a submission may be found in an international agreement).
Relying on previous case law concerning waivers of state immunity, the UKSC first noted that the test for whether there had been a submission for the purposes of s.2(2) SIA was whether there had been a clear and unequivocal expression of the state’s consent to the exercise of the jurisdiction. This did not require the use of explicit words such as "waiver" or "submission"; rather, the question is whether the words used necessarily lead to the conclusion that the state has submitted to the jurisdiction [69].
Having established that, the UKSC interpreted Article 54 in accordance with the Vienna Convention on the Law of Treaties (VCLT) to ascertain whether the test was met. Article 54(1) obliges each Contracting State to recognise ICSID awards as binding and to enforce them as if they were a domestic final judgment. Applying the principles under Article 31 VCLT, the UKSC held that the ordinary meaning of Article 54(1) was that any given Contracting State not only agreed that it would recognise and enforce ICSID awards but that, in doing so, it also necessarily agreed to other Contracting Parties doing the same, even in situations where the ICSID award in question was against it. So, that necessarily involved a waiver of its adjudicative immunity otherwise the obligation is no longer mutual and reciprocal [82,89]. The UKSC held that this meaning was also clear from the fact that, for instance, Article 55 preserved state immunity from execution but not from adjudicative jurisdiction [88]. The UKSC further held that this interpretation was consistent with the object and purpose of the ICSID Convention, which it found was to encourage the flow of private investment by offering assurances against sovereign risk, with the overarching goal of producing binding awards supported by a fully reciprocal regime to ensure compliance. The UKSC’s conclusion was, therefore, that Article 54(1) did, in respect of Contracting States (including Spain and Zimbabwe) constitute, under s.2(2) SIA, a clear and unequivocal submission to the adjudicative jurisdiction of the English courts for the purpose of registering ICSID awards [118,144].
Since the meaning was clear under Article 31 VCLT, recourse to the travaux préparatoires under Article 32 was not strictly necessary, but the UKSC considered that, in any event, they were also supportive of its conclusion [119-133]. It also examined the approaches taken by other jurisdictions and found them to be largely consistent in holding that Article 54(1) waived adjudicative immunity in this way [134-142].
Given the UKSC’s conclusion on the above, it did not have to decide on the point concerning the operation of s.9 SIA, which the UKSC left open [145].
The Court’s decision helps clear the path for investors seeking registration of ICSID awards in the UK by curtailing challenges from contracting states on the basis of sovereign immunity. Since the waiver of adjudicative immunity is established under s.2 SIA, there is no need for the investor to rely on s.9 SIA. This means that objections relating to the underlying arbitration agreement (for instance, Spain’s challenge to its validity in this case) cannot be raised/relitigated as a basis for resisting registration. The ruling, however, is limited to immunity from adjudicative jurisdiction / the registration process and does not affect the extent of states' sovereign immunity with regard to execution (i.e., the actual process of recovery against state-owned assets to which, in the UK, the provisions of s.13 SIA would apply).
The Linklaters International Arbitration Group has significant experience in dealing with matters concerning the recognition and enforcement of treaty awards, including of the type raised by this case. To discuss further, please contact any of our team shown to the left, or your usual Linklaters contact.
8 April 2026