To prove most criminal offences, it must be shown that the actus reus (action) and mens rea (intention) are present. In order to prove an offence of ABH for example, it is necessary to show that a) the individual did the act which caused actual bodily harm (the assault), and b) either intended to cause that injury by doing that action, or was reckless as to the consequences of the assault.
The reason for this is that it is unpalatable to criminalise an act or omission where the individual does not possess any intention for the act to come about. For example, it would be utterly wrong to be able to convict someone of an offence of theft without needing to prove that they were dishonest.
Strict liability offences are offences where mens rea does not form a part of what it is necessary to prove the offence. These are offences where there is no mens rea required to establish liability, in relation to one or more elements of the actus reus of the offence; in short, the ‘guilty act’ is sufficient. This is different from absolute liability which is where liability is imposed irrespective of mens rea, and without the availability of any defence.
A good example may be driving with excess alcohol. When a motorist provides a sample of blood or breath which is in excess of the prescribed limit, it is only necessary to prove that they had alcohol in their blood/breath in excess of the limit. It is not necessary to prove that they intentionally had so, or were reckless as to being over the limit. Imagine if it were necessary to prove a ‘guilty mind’ for that offence; there would be far fewer successful prosecutions.
Strict liability can be determined by looking at the intention of Parliament. If it is decided that Parliament purposefully left out the mens rea of an offence, it is treated as strict liability. Where, however, it appears Parliament remained silent on the issue of mens rea, and it does not appear it was purposefully left out, the presumption is that there is a mental element of the offence required to be proven.
Despite the potential to create injustice, Parliament has deemed it necessary to make certain offences strict liability for practical reasons (drink driving, speeding etc.) or where additional protection is deemed necessary (certain firearms offences).
A strict liability offence removes the availability of the defence of diminished responsibility, honest belief and mistake.
The upshot is that a person charged with a strict liability offence will need to show that the actus reus is not made out – for example, for speeding, that they were not in fact speeding (the speed gun recorded an inaccurate reading) or that they weren’t driving at the relevant time.
Defences such as self defence will not be relevant because they go to the mental state of the defendant at the relevant time.
Strict liability does not necessarily infringe the presumption of innocence under Article 6(2), see R v Lambert 2001 UKHL 37.
See also Salabiaku v France (1988) 13 EHRR 379, ECtHR and Barnfather v Islington London Borough Council  EWHC 418 (Admin).
R v Howells 1977 QB 614
The defendant had a firearm without a certificate. He claimed that it was an antique, and if it was not an antique, his honest belief that it was an antique should provide him a defence to the offence of possession of a firearm without a certificate (Firearms act 1968 s 1(1)(a)). The jury were directed to convict the defendant if they found that the firearm was not an antique.
He appealed. The Court of Appeal held that the offence was to be read ‘strictly’, and so mens rea was not necessary. The jury were properly directed and the conviction was upheld.