What authority has been sought to determine whether to sever an unconstitutional statutory provision instead of striking down the entire section?

Counsel agree that the leading authority on determining whether to sever an unconstitutional statutory provision instead of striking down the entire section is Schachter v. Canada, 1992 CanLII 74 (SCC), [1992] 2 S.C.R. 679, [1992] S.C.J. No. 68 [Cited to QL]. In summary, the judgment holds that laws have always been struck down only to the extent of the inconsistency using the doctrine of severance or ‘reading down’ (at para. 26). Severance is used “so as to interfere with the laws adopted by the legislature as little as possible” (at para. 26). When only part of the legislation violates the Constitution, “only the offending portion should be declared to be of no force or effect, and the rest should be spared” (at para. 26). In that way, it allows as much of the legislative purpose as possible to be realized (at para. 29). When only a portion of a statute violates the Constitution, it is common sense that the offending portion should be declared of no force or effect and the rest spared (at para. 26).