"x x x.
While it is true that petitioner had been asking the rehabilitation and appellate courts that it be allowed to participate, contrary to respondents’ contention, the same did not amount to estoppel that would bar it from questioning the rehabilitation court’s jurisdiction. It is well-settled that the court’s jurisdiction may be assailed at any stage of the proceedings, even for the first time on appeal. The reason is that jurisdiction is conferred by law, and lack of it affects the very authority of the court to take cognizance of and to render judgment on the action. In its Opposition to the petition for rehabilitation, petitioner already questioned the court’s jurisdiction over UDI. On appeal to the CA, it again raised the same issue, but it failed to obtain a favorable decision. We cannot, therefore, say that petitioner slept on its rights. It is not estopped from raising the jurisdictional issue even at this stage. In any event, even if petitioner had not raised the issue of jurisdiction, the reviewing court would still not be precluded from ruling on the matter of jurisdiction.
Neither can estoppel be imputed to petitioner for its receipt of payments made by respondents in accordance with the rehabilitation plan. It has been established that in its letters to respondents, petitioner explained that it received payments subject to the results of its appeal. Besides, it is a basic rule that estoppel does not confer jurisdiction on a tribunal that has none over the cause of action or subject matter of the case.
x x x."