The plea of arbitration agreement raised by the defendant in ordinary proceedings does not automatically determine the outcome of the dispute in favour of a declaration declining jurisdiction in favour of the arbitrators, it being necessary to assess in concreto the objective scope of the arbitration clause in relation to the claim brought, so that the legal representative's choice to proceed with voluntary intervention as a supporting party pending such plea does not in itself constitute professional breach, in the absence of proof of the causal link between the conduct and the prejudice suffered by the client.