366. Prohibition of retrospective punishment in English common law.

The common law presumes that statutes are not intended to have retrospective effect1. In 1946 it was observed that 'a man is not to be put in peril upon an ambiguity'2, and more recently it has been indicated that the common law imposes a general prohibition of the retrospective application of the criminal law3. The bases for the presumption against retrospectivity include the inability to foresee that one's conduct would be treated as criminal when it occurred4, and the principle that Parliament cannot be taken to have altered the law applicable to past events in a manner which is