Under Article 34(2) of Legislative Decree No. 5/2003 (applicable ratione temporis), the arbitration clause must confer the power to appoint all arbitrators upon a person external to the company, on pain of nullity. Partial nullity under Article 1419 of the Civil Code cannot save the clause where the removal of the invalid part would deprive it of meaning. The absence of mandatory rules capable of substituting the invalid provision entails the total inefficacy of the arbitration clause.