25.—Where persons F45[(being either spouses of each other or persons who were formerly spouses of each other)] are charged in the same proceedings, neither shall at the trial be competent by virtue of section 21 (a) to give evidence at the instance of the prosecution, or be compellable by virtue of section 22, 23 or 24 to give evidence, unless the person concerned is not, or is no longer, liable to be convicted at the trial as a result of pleading guilty or for any other reason.