Our article on the extension of arbitration agreement to non-signatory third parties under Spanish law

The extension of an arbitration agreement to non-signatory third parties gives rise to a structural tension between the principle of consent –which lies at the core of arbitration as a qualified expression of party autonomy– and the need to ensure the effectiveness of the arbitration agreement against avoidance strategies based on corporate structures or complex contractual arrangements. Spanish law has addressed this tension by combining three distinct dimensions: the constitutional requirements governing the validity of arbitration agreements –Article 24 of the Spanish Constitution (Constitución Española, “CE”), together with Constitutional Court Judgments (CCJ) 1/2018 of 11 January and 146/2024 of 2 December; the statutory framework established by Arbitration Act 60/2003 of 23 December (the “Arbitration Act” or “AA”) –particularly Articles 9 (arbitration agreement), 22 (the principle of Kompetenz-Kompetenz), and 41 (the action for setting aside arbitral awards)–; and the case law developing doctrines such as the group of companies, piercing the corporate veil, and material participation in the contractual relationship. CCJ 1/2018 of 11 January held that arbitration is constitutionally legitimate only where it is based on the free and mutual consent of the parties; consequently, the statutory imposition of arbitration upon a person who has not agreed to submit to it infringes Article 24(1) CE. In turn, CCJ 146/2024, in defining the scope of judicial review of arbitral awards on grounds of public policy, confirmed that only violations affecting fundamental rights or essential principles of the constitutional public order may justify setting aside an arbitral award.

Against this background, the extension of the arbitration agreement is conceived as an exceptional corrective mechanism applicable where the economic reality and the parties’ conduct demonstrate that certain persons, although they have not formally signed the arbitration clause, have acted and benefited as genuine parties to the arbitration agreement. The result is a model in which consent remains the governing principle but is interpreted substantively rather than merely formally, with case law serving to refine and apply the relevant criteria rather than constituting the sole source of the doctrine. Since Supreme Court Judgment (SCJ) 404/2005 of 26 May, Spanish case law has recognised, in exceptional circumstances, the “transmission” or subjective extension of an arbitration agreement to third parties directly involved in the performance of the contract containing the arbitration clause, even where they have not signed it, provided that the overall circumstances reveal a substantive intention to submit to arbitration or conduct inconsistent with denying such submission.

A particularly significant role in this development has been played by the recent case law of the High Courts of Justice (Tribunales Superiores de Justicia), most notably High Court of Justice of Aragon Judgment 17/2025 of 12 December, which exemplifies the growing tendency to restrict judicial review of arbitral awards while, at the same time, upholding arbitral decisions extending arbitration agreements to non-signatory third parties on the basis of the doctrine of piercing the corporate veil and their participation in the performance of the contract. This demonstrates that the extension of arbitration agreements is not treated as an anomaly but rather as a necessary mechanism to prevent the formal structure of a corporate group or transactional arrangement from depriving the arbitration agreement of its intended effect.

You can access the full article (in Spanish) through this link*.

*English language version available upon request.
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