Section 34B. In any civil action where twelve jurors or more are impaneled, if a juror dies, or becomes ill, or is otherwise unable to perform his duty for good cause, the trial shall proceed; provided, however, that except by agreement of the parties, no such civil jury trial shall proceed with less than ten jurors remaining on the panel.

Nothing in this section shall impair the right of the court to fill vacancies in the manner provided in section twenty-six B.