An estoppel is a legal doctrine that prevents a party from relying upon certain legal rights or facts where such reliance may be unconscionable.1 It is a flexible doctrine, both in terms of the circumstances, which may fall within its purview, and the breadth of relief a tribunal or court may award to satisfy any equity that might arise.
It has been given wide international recognition2 and its principles are similarly applicable in the realm of international law where it is considered that sovereign States and private parties ought to be consistent in their attitude to a given factual or legal situation irrespective of its truth.3
Historically, the principle of estoppel was borne out of a common law desire to prevent an unjust departure by a party from an assumption adopted by another as the basis of some act or omission which, unless the assumption is adopted, would operate to that party’s detriment.4 "Whether a departure by a party from the assumption should be considered unjust and inadmissible depends on the part taken by him in occasioning its adoption by the other party."5
In the context of investment arbitration, scholars and tribunals have both underlined the relationship entertained by the principle of estoppel with other notions of international law and standards of treatment. Links between the notions of estoppel and legitimate expectations have been underlined.6 Similarly, it has been implied that claims under the doctrine of estoppel could overlap with those raised under the standard of fair and equitable treatment.7
Estoppel is to be distinguished from the doctrine of collateral estoppel, similarly derived from common law.8 Collateral estoppel, while associated with the notion of res judicata,9 prevents parties from litigating an issue already effectively decided in a previous proceeding.10 See further res judicata.
An estoppel may arise when a dispute involves one of the parties making some form of representation by words or by conduct acknowledging a state of affairs. The party is thereby precluded (or “estopped”) from asserting that the opposite position was true in law or in fact, whether or not it actually was.
Similarly, in the international investment law context, estoppel could be characterised as a legal response to prevent inconsistent behaviour,11 but not to create rights.12 It is recognized under international law through four doctrinal concepts: recognition, acquiescence, waiver and estoppel.13 These are rooted in the concept of good faith.14 The interferences between the two notions (estoppel and bona fide) have led to the clear finding that the party against whom estoppel is raised ought to have acted in bad faith for the doctrine to apply.15
In cases where parties relied on estoppel to bar or enlarge the tribunal’s jurisdiction, it was dealt with as a jurisdictional question.18 For instance, tribunals barred parties from bringing jurisdictional objections where they asserted throughout the proceedings that they would refrain from doing so.19 Tribunals also refused to allow estoppel to create jurisdiction,20 particularly where the requirements of Article 25 of the Washington Convention were not met.21
The “strict view” purports that an estoppel arises where:27
One of the first expressions of the doctrine of estoppel can be found in early ICSID cases such as Amco v Indonesia37 which adopted this strict approach. Subsequently, it has been largely premised upon tribunals’ application of the issue.38 This is also the prevailing view in public international law.39
It has been argued that the “broad view” should be adopted because inconsistent behaviour is not to be tolerated and a “State [or party] cannot adopt inconsistent positions in respect of the same state of facts”.41 It has also been argued that the “broad view” encompasses public international law’s recognition of general conceptions of fairness42 and good faith.43 However, it remains that the “broad view” is not the prevailing position. The estoppel doctrine has always required some form of adherence to a particular framework and detriment of some description.
In comparison to common law gestations of estoppel, the international law version of the doctrine is sometimes considered to be imprecise and variable. Additionally, while most tribunals have adopted the “strict view” approach, a few others have endorsed the “broad view” without citing any authority.44
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