In order to decide whether an offence is one of strict liability, the courts start by assuming that mens rea is required, but they are prepared to interpret the offence as one of strict liability if Parliament has expressly or by implication indicated this in the relevant statute.
The judges often have difficulty in deciding whether an offence is one of strict laibility or not. The first rule is that where an Act of Parliament includes words indicating mens rea (such as 'knowingly', 'intentionally', 'maliciously' or 'permitting'), the offence requires mens rea and is not one of strict liability. However, if an Act of Parliament makes it clear that mens rea is not required, the offence will be one of strict liability.
However, in many instances a section in an Act of Parliament is silent about the need for mens rea. Parliament is criticised for this. If it made clear in all sections which create a criminal offence whether mens rea was required or not, then there would be no problem. As it is, where there are no express words indicating mens rea or strict liability, the courts have to decide which offences are ones of strict liability.
Where an Act of Parliament does not include any words indicating mens rea, the judges will start by presuming that all criminal offences require mens rea. This was made clear in the case of Sweet v. Parsley (1969).