"As a general rule an honest and reasonable belief in a state of facts which, if they existed, would make the defendant's act innocent affords an excuse for doing what would otherwise be an offence.
The strength of the presumption that the rule applies to a statutory offence newly created varies with the nature of the offence and the scope of the statute. If the purpose of the statute is to add a new crime to the general criminal law, it is natural to suppose that it is to be read subject to the general principles according to which that law is administered. But other considerations arise where in matters of police, of health, of safety or the like the legislature adopts penal measures in order to cast on the individual the responsibility of so conducting his affairs that the general welfare will not be prejudiced. In such cases there is less ground, either in reason or in
actual probability, for presuming an intention that the general rule
should apply making honest and reasonable mistake a ground
of exoneration, and the presumption is but a weak one.
Indeed, there has been a marked and growing tendency to treat the prima facie rule as excluded or rebutted in the case of summary offences created by modern statutes, particularly those dealing with social and industrial regulation. But, although it has been said that in construing a modern statute a presumption as to mens rea does not exist (per Kennedy L.J., Hobbs v. Winchester Corporation), it is probably still true that, unless from the words, context, subject matter, or general nature of the enactment some reason to the contrary appears, you are to treat honest and reasonable mistake as a ground of exculpation, even from a summary offence."