MISTAKE OF LAW
There is a general rule that ignorance of the criminal law is no defence, even if the ignorance is reasonable in the circumstances.
By way of contrast to mistake of criminal law, mistake of civil law can quite easily provide a defence to a criminal charge, provided the actus reus of the offence involves proof of a legal concept. See:
R v Smith  QB 354. The defendant was the tenant of a flat. With the landlord's consent he installed some hi-fi equipment and soundproofing. When given notice to quit the flat, the defendant tore down the soundproofing to remove some wires that lay behind. Unknown to the defendant the soundproofing had, as a matter of civil law, become a fixture of the property and therefore property belonging to the landlord. The defendant's conviction for criminal damage was quashed by the Court of Appeal. It was held that no offence is committed if a person destroys or damages property belonging to another if he does so in the honest though mistaken belief that the property is his own.
MISTAKE OF FACT
A) MENS REA OFFENCES
It is clear from that a mistake of fact, rather than law, is a defence where it prevents the defendant from forming the mens rea which the law requires for the crime with which he is charged.
Where the law requires only negligence, then only a reasonable mistake will
lead to a defence: an unreasonable mistake is one which a reasonable man would
not make and is, therefore, negligent.
C) STRICT LIABILITY OFFENCES
If no mens rea is required with regard to one element of the actus reus then even an honest and reasonable mistake with regard to that element will not negative liability. For example, see:
R v Prince (1875). The defendant took a girl under 16, believing on reasonable grounds that she was 18, out of the possession of her parents. The defendant was convicted and the conviction affirmed. (See Handout on Strict Liability.)
D) DRUNKEN MISTAKE
Where a defendant's mistake of fact arises from self-induced intoxication, it will only provide a defence to crimes of specific intent. In general, where a defendant is charged with a basic intent crime, the jury will be directed that evidence of self-induced intoxication is irrelevant to the question of what the defendant believed to be happening. See for example:
R v Woods (1981) 74 Cr App R 312. The defendant was convicted of a collective rape of one girl. He made admissions of his part in it to the police but at his trial he went back on those admissions and said in effect that he had so much to drink that he was not sure what had happened. He did not know whether he had raped her or not and did not realise that she was not consenting to anything that went on. Griffiths LJ stated: "The law, as a matter of social policy, has declared that self-induced intoxication is not a legally relevant matter to be taken into account in deciding as to whether or not a woman consents to intercourse."
R v Fotheringham  Crim LR 846. The defendant got into his own bed after coming home from a party and forced the baby-sitter (who was already in the bed) to have sexual intercourse. He claimed that he was so drunk that he thought the girl was his wife. The Court of Appeal upheld his conviction. It was held that (1) self-induced intoxication cannot be used as a defence to a crime of basic intent and stated that (2) neither could the defence of mistake be raised, if this mistake were caused by self-induced intoxication: R v O'Grady  3 WLR 321.
E) BURDEN OF PROOF
Whilst there is always an evidential burden on the defendant to put evidence before the jury that he did actually make the mistake upon which he relies, the legal burden always rests with the prosecution to establish beyond reasonable doubt that the defendant was not mistaken and therefore did have the requisite mens rea for the offence with which he is charged.
As is the case in any trial where the prosecution fails to establish mens rea, if the defendant succeeds with his defence of mistake he must be acquitted.
See also: handouts on Intoxication and Self-defence.