The plea that a dispute has been referred to contractual arbitration (arbitrato irrituale), amounting to a waiver of judicial proceedings and of State jurisdiction through the choice of a mechanism of a private-law nature, raises a question on the merits concerning the assertion of a fact precluding the exercise of State jurisdiction and constitutes a plea in the strict sense, with the result that it must be raised by the parties within the time limits and in the manner prescribed for pleas on the merits, on pain of preclusion.