Where a prospectus, issued, circulated or distributed under this Chapter, includes any statement which is untrue or misleading in form or context in which it is included or where any inclusion or omission of any matter is likely to mislead, every person who authorises the issue of such prospectus shall be liable under section 447:
     Provided that nothing in this section shall apply to a person if he proves that such statement or omission was immaterial or that he had reasonable grounds to believe, and did up to the time of issue of the prospectus believe, that the statement was true or the inclusion or omission was necessary.