Third-party opposition pursuant to Article 404, paragraph 1, of the Code of Civil Procedure, as referred to by Article 831, paragraph 3, of the Code of Civil Procedure in arbitral matters, may be brought exclusively by persons who, whilst not having been parties to the arbitral proceedings, claim an autonomous right incompatible with the legal relationship established by the award and are prejudiced thereby.
In third-party opposition against an arbitral award, where the right established against the opponent is absolutely incompatible with that recognised by the challenged award, the judgment allowing the opposition must rule both in its rescissory capacity, declaring the award ineffective against the third party, and in its substitutive capacity, establishing the true nature of the right asserted by the opponent.
In third-party opposition against an arbitral award, the burden of proving the right incompatible with that established by the award rests entirely upon the opponent, who must demonstrate ownership of the claimed right by means of specific documentary or testimonial evidence, it not being sufficient to demonstrate the mere non-existence of the prerequisites for the determination contained in the award.