The arbitration clause must be drafted in writing on pain of nullity only when the arbitration concerns disputes relating to legal relationships for which written form is required ad substantiam, whilst when it concerns disputes relating to other relationships, the conferment of mandate to arbitrators must only be proven in writing according to the rules laid down by Article 1967 of the Civil Code. Written proof may be constituted by any written attestation of the existence of the arbitral mandate, even if subsequent to the agreements and of merely recognitory character, provided it is attributable to the parties.
The lack of specific written approval of the arbitration clause for contractual arbitration (arbitrato irrituale) does not determine its invalidity, it being necessary to exclude that such clause falls among those to be specifically approved in writing pursuant to Articles 1341 and 1342 of the Civil Code, not having content derogating from the jurisdiction of the ordinary judge, unlike that for regular arbitration. This applies even when the discipline of the arbitral proceeding is established by reference to regulations prepared by third parties and incorporated in the contract, with consequent burden on both parties to acquire knowledge thereof.
The arbitration clause which provides for recourse to arbitration for dispute resolution does not preclude recourse to the ordinary judge to demand payment of invoices relating to goods received without contestation, when such exception is expressly provided in the general contract conditions.