There is no denying that there is a serious backlash against investment arbitration at the moment. The signs are everywhere: from the latest discussions occurring within UNCITRAL’s Working Group III to the more recent practice of states (see the 22 European Union Member States’ declaration concerning the termination of their intra-EU Bilateral Investment Treaties); the latest ‘battlefront’ seems to be the Energy Charter Treaty, where the investment tribunals seized of disputes on the basis of this treaty consider it immune from the effects of the Achmea decision. The causes for this backlash are manifold. For present purposes, however, I would like to focus my attention on only one of the causes: incorrect decisions. And I would like to go even further and look at a very specific example of incorrect decisions: the application of the principle of estoppel by investment tribunals. I will focus exclusively on the procedural aspect of estoppel, as a bar to a claim. This seems to be its main, although not its only (para. 831), function – at least in international investment law.
Some background information on estoppel
Generally, estoppel is a very strong mechanism which has a preclusive effect against a party contradicting itself if another party has relied (usually to the latter’s detriment) on the initial position of the former (para. 231). Essentially, the party which contradicts itself is prevented from averring the contradictory fact (the subsequent one). ‘[W]hat is relevant for estoppel is that there has been a declaration, representation, or conduct which has in fact induced reasonable reliance by a third party, which means that the State, even if only implicitly, has committed not to change its course’ (idem, para. 246). Furthermore, the element which induces reliance must be unambiguous (paras. 8.46-8.47). Other tribunals refer to the fact that representations must be ‘clear and consistent’ (for example, the Chagos Marine Protected Area Arbitration, para. 438).
In international law, the application of estoppel dates back to the days of the Permanent Court of International Justice: for example, in the Legal Status of Eastern Greenland case, Norway was precluded from asserting sovereignty over Greenland, as the former had expressly recognized the latter as part of Denmark. This form of estoppel, however, seems to heavily overlap with vaguer principles – including the principle of good faith (para. 483).
There are voices in international law which argue that estoppel as such exists in a single form in international law and not in its various iterations found in the domestic common law systems (para. 436). This view, however, is not shared by all international law practitioners. Whether due to fragmentation of international law or not, this divergence becomes obvious once one analyzes arbitral practice. One example of how arbitral tribunals have looked at estoppel in its specific iterations concerns procedural aspects. There, estoppel acts as a more specific and technical mechanism designed to prevent an already litigated claim from being pursued again (similar to res judicata, although with a few important differences which will be mentioned below). The important branches of estoppel which may preclude a claim from being relitigated are: cause of action estoppel; and issue (or collateral) estoppel. It is important to mention that both these doctrines ‘prevent the parties from re-litigating a question that has been determined by a Court of competent jurisdiction, between the same parties or their privies, in a previous action. Once those elements have been made out, and unless there are special circumstances, the parties are precluded from raising the issues. [footnote omitted] The special circumstances which would permit the issue to be raised again include the discovery of further material relevant to issues in the first set of proceedings [footnote omitted] or fraud’. The essential difference between the two doctrines, according to Griffith and Seif, is that cause of action estoppel concerns the claim itself which is precluded, whereas issue estoppel prevents relitigation of a point of law or of fact already decided by a tribunal. Wilken QC and Ghaly point out that the difference is one of specificity. According to them, ‘issue estoppel bites on the facts and issues required to establish the cause of action whereas cause of action estoppel looks only at the cause of action’. Sheppard equates ‘cause of action’ with ‘claim’.
A very important point of difference between estoppel – in both its iterations – and res judicata is that the latter requires (at least traditionally, as Judge Anzilotti mentioned in his dissenting opinion to the Factory at Chorzów case) a three-element identity between the concerned claims (the same person, the same claim and the same legal grounds); also known as the ‘three-element test’. Moreover, estoppel extends to the privies of the relevant parties, while res judicata – if interpreted strictly – does not. Without going into the details of how the three elements of res judicata have been interpreted, especially in investment arbitration (as this is another subject for another date), it can be reasonably stated that estoppel is a stronger tool (than res judicata) in the arsenal of investment tribunals which can be used to prevent abusive re-litigation. The problem, however, is that the existence of such an instrument in international law is not clearly evident and tribunals seem to have taken its existence for granted.
The problems with the investment tribunals’ application of estoppel
Although not a general principle of law, some arbitral tribunals seem to have applied estoppel as such. As will be seen below, however, there is at best inconclusive evidence as to the existence of a general principle of estoppel and at worst clear attempts to disregard this non-existence and apply a principle out of nothing.
At the same time, there are arbitral tribunals which may suggest or clearly determine that estoppel is a principle of law, although this is usually not explained clearly and the reasoning is incomplete. As such, one is left wondering how did the tribunal uncover such a principle and whether it really exists.
For example, the Petrobart tribunal mentioned that ‘while the doctrine of collateral estoppel seems to have primarily developed in American law, other legal systems have similar rules which in some circumstances preclude examination of an issue which could have been raised, but was not raised, in previous proceedings. A doctrine of estoppel is also recognised in public international law’ (at pp. 66-67).
The tribunal, however, was unclear whether this amounted to a principle of law or not. The fact that there exist rules which establish preclusion of issues which could have been raised but were not raised and that these rules occur outside of the American legal system, as well, does not transform estoppel into a principle of law. At the same time, the tribunal did not mention in what form is estoppel recognised in public international law. It may have suggested that this would be applied as a principle, but it stopped short of fully clarifying whether such a principle indeed exists. The alternative may have been the customary law nature of estoppel, but the tribunal neither identified the underlying state practice and opinio juris nor referred to awards/ judgements in which such a custom was established. In the end, the claim preclusion argument was anyway rejected, since – among others – there was no identity between the legal grounds relied on in the relevant proceedings (at pp. 67-68).
Another example is RSM v. Grenada. There, the tribunal explicitly endorsed collateral estoppel as a general principle of law (para. 7.1.2). The tribunal noted ‘that the doctrine of collateral estoppel is now well established as a general principle of law applicable in the international courts and tribunals such as this one’. [footnote omitted] (ibid.). However, it did not come to this conclusion itself, but rather relied on other tribunals’ conclusions. What is surprising after looking at the cited cases is that neither of them clearly endorses estoppel as a principle of law.
For example, the Amco v. Indonesia tribunal referred to res judicata as a principle of law (paras. 26-46). One cannot exclude the possibility of this encapsulating estoppel as well, but such a conclusion is not clear. This lack of clarity is further compounded by the fact that the Amco v. Indonesia tribunal mentioned that ‘it is by no means clear that the basic trend in international law is to accept reasoning, preliminary or incidental determinations as part of what constitutes res judicata’ (idem, para. 32). As issue/collateral estoppel necessarily implies the fact that the reasoning of an award must be considered for this mechanism to arise, the finding of the Amco v. Indonesia tribunal raises serious doubts as to the conclusion that estoppel was part of the principle to which that tribunal referred.
As regards the other relevant case (Southern Pacific Railroad Co. v. United States, which arose before the Supreme Court of the United States) it is true that what the cited tribunal referred to was issue estoppel (pp. 48-49). It mentioned that a general principle existed which mandated ‘that a right, question, or fact distinctly put in issue, and directly determined by a court of competent jurisdiction as a ground of recovery cannot be disputed in a subsequent suit between the same parties or their privies, and, even if the second suit is for a different cause of action, the right, question, or fact once so determined must, as between the same parties or their privies, be taken as conclusively established so long as the judgment in the first suit remains unmodified’ (ibid.). What the tribunal does not mention, however, is whether this general principle is a general principle common to all nations or whether this was a general principle specific only to the common law system.
There are tribunals which even seem to rely on estoppel, although, in reality, they are applying res judicata. This was the case with the Marco Gavazzi and Stefano Gavazzi v. Romania tribunal (paras. 164-166). In the first place, the tribunal analyzed whether an initial decision (which was alleged to preclude the claims before the forum) had ‘conclusive effects on the Parties to the present proceedings under the doctrine of res judicata or issue estoppel’ (idem, para. 164). Subsequently, it went on to mention that ‘under international law, three conditions need to be fulfilled for a decision to have binding effect in later proceedings: namely, that in both instances, the object of the claim, the cause of action, and the parties are identical’ (idem, para. 166). Although it did expressly refer to issue estoppel at one point, the tribunal referred to the conditions which were necessary to be fulfilled in order for res judicata to operate (the three-element test, as mentioned above). Moreover, it conflated issue estoppel with cause of action estoppel. As shown earlier, identity of cause of action is only necessary in the case of cause of action estoppel and not in the case of issue estoppel.
All the above examples demonstrate that estoppel as such is not applicable in investment arbitration (by virtue of international law, at least) and that tribunals seem to ignore this. There is no general principle – as understood by Article 38 (1) (c) of the Statute of the International Court of Justice, as an authoritative reflection of the sources of international law – of estoppel. At least no principle which could cover cause of action or issue estoppel. There is no evidence of a customary rule encapsulating estoppel either. Moreover, not even investment treaties seem to contain this mechanism. For example, the 2012 US Model BIT – selected for being relevant to a common law jurisdiction – does not make any reference to estoppel. Neither does one of the latest UK BITs (the UK-Colombia BIT) contain any reference to estoppel – although it does allow the tribunal to address abuse of process; however, this is different than estoppel.
 Editor at avocatnet.ro and Associate Expert at DAVA | Strategic Analysis. This post is based on part of my thesis, submitted for the completion of an LL. M. in Law and Economics at Utrecht University. I would like to express my gratitude to Dr. Yulia Levashova, for her continuous support and for an in-depth and comprehensive feedback. In any case, I take full responsibility for the opinions and they are exclusively mine, not reflecting anyone else’s or any other institution’s.
 Audley Sheppard, ‘Chapter 8. Res Judicata and Estoppel’ in Bernardo M. Cremades Sanz-Pastor and Julian D.M. Lew (eds.), Parallel State and Arbitral Procedures in International Arbitration, p. 225 (hereinafter referred to as ‘Sheppard’).
 Sean Wilken QC, Karim Ghaly, The Law of Waiver, Variation and Estoppel. Third Edition (Oxford University Press 2012), para. 14.08 (hereinafter referred to as Wilken QC, Ghaly).
 Gavan Griffith; Isabella Seif, ‘Chapter 8: Work in Progress: Res Judicata and Issue Estoppel in Investment Arbitration’, in Neil Kaplan and Michael J. Moser (eds), Jurisdiction, Admissibility and Choice of Law in International Arbitration: Liber Amicorum Michael Pryles (Kluwer Law International 2018), p. 124 (hereinafter referred to as ‘Griffith; Seif’).
 Wilken QC, Ghaly, para. 14.09.
 Sheppard, p. 225.
 Griffith; Seif, p. 126.
 Charles T. Kotuby Jr. and Luke A. Sobota, General Principles of Law and International Due Process. Principles and Norms Applicable in Transnational Disputes (Oxford University Press 2017), footnote 262, p. 200. Such a conclusion (that estoppel is not a general principle of law) is in accordance with one of the major views in international legal relations as to what constitutes a general principle of law: one ‘which can be derived from a comparison of the various systems of municipal law, and the extraction of such principles as appear to be shared by all, or a majority, of them [emphasis added]’, Hugh Thirlway, The Sources of International Law. Second Edition (Oxford University Press, 2019), p. 108.
 Stating that estoppel is a principle of law serves two aims: firstly, the tribunal justifies the application of estoppel by reference to a source of international law (usually, part of the applicable law). Secondly, this gives the tribunal legitimacy, as the tribunal grounds its decision to rely on estoppel on a widely-applicable source of law (whether objectively true or not is not as important).
 The cases to which the RSM tribunal referred were mentioned at page 27, footnote 34 of the award: Amco Asia Corporation v Republic of Indonesia, ICSID Case No. ARB/81/1, Decision on Jurisdiction (Resubmitted Case), 10 May 1988, para. 30; Company General of the Orinoco Case, 10 R.I.A.A. 184 (1905); and Southern Pacific Railroad Co. v. United States, 168 U.S. 1 (1897). The second tribunal quoted in turn the third one. As such, I will refer only to the first and third tribunals in the remainder of this part.
 Sheppard, p. 234; Griffith, Seif, p. 121.
 Christopher Brown, ‘A Comparative and Critical Assessment of Estoppel in International Law’, University of Miami Law Review [Vol. 50:369 1996], pp. 384-385;Pan Kaijun, ‘A Re-Examination of Estoppel in International Jurisprudence’, 16 Chinese Journal of International Law (2017), p. 761.