Whenever in the opinion of the court the trial of any criminal or civil case is likely to be a protracted one, the court may direct the selection of additional jurors who shall be drawn from the same source, in the same manner and at the same time as the regular jurors. These additional jurors shall have the same qualifications, and be considered and treated in every respect as regular jurors and be subject to examination and challenge as such jurors. When one additional juror is desired, there shall be drawn three veniremen, and the plaintiff and defendant in a civil case or the Commonwealth and accused in a criminal case shall each be allowed one peremptory challenge. When two or more additional jurors are desired there shall be drawn twice as many venireman as the number of additional jurors desired. The plaintiff and defendant in a civil case or the Commonwealth and accused in a criminal case shall each be allowed one additional peremptory challenge for every two additional jurors. The court shall select, by lot, those jurors to be designated additional jurors. The plaintiff and defendant in a civil case or the Commonwealth and accused in a criminal case shall be advised by the court which jurors are additional jurors at the time the jury is impaneled; however, in no event, shall any juror be made aware of his status as a regular or additional juror until he is excused as a juror. Before final submission of the case, the court shall excuse any additional jurors in order to reduce the number of jurors to that required by §§ 8.01-359 and 19.2-262.