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Defence of Mistake
Where the defendant acts under a mistaken belief of the circumstances they may be afforded the defence of mistake. Where a defendant acts under such a mistake, the mistake prevents them forming the mens rea of the crime and thus mistake is not really a defence as such, but relates to the absence of the elements of establishing liability. The defence of mistake was first recognised in R v Tolson (1889) 23 QBD 168 but has developed since then. Often the defence of mistake is complicated by being combined with other defences such as intoxication or self-defence (or both).
Initially a defence would be allowed if the mistake was both honest and reasonably held: 
R v Tolson (1889) 23 QBD 168  Case summary
A mistake as to law will not generally suffice, for the defence of mistake, since ignorance of the law is no excuse (Ignorantia juris non excusat)
R v Lee [2000] EWCA Crim 53    Case summary
Although a mistake of civil law may be sufficient to find a defence of mistake:
R v Smith [1974] QB 354   Case summary
A mistake of fact will suffice provided the mistake was such as to prevent the defendant forming the mens rea of the offence. Whilst initially the mistake was required to be both honest and reasonably held, in DPP v Morgan the House of Lords held that the mistake need only be honest. There was no requirement that it was reasonable for the defendant to make the mistake:
DPP v Morgan [1976] A.C. 182  Case summary
The defence of mistake in relation to public/private defence
The defence of mistake may be raised in conjunction with self-defence and prevention of crime where for example the defendant mistakenly believed he was under attack or using reasonable force in the prevention of crime. The same principle applies that the mistake must be honest, but need not be reasonably held:
R v Gladstone Williams (1984) 78 Cr App R 276  Case summary
Beckford [1987] 3 WLR 611  Case summary

Implications where the mistake is induced by intoxication
Where the mistake is induced by voluntary intoxication, and the crime is one of basic intent, the defendant is not allowed the defence of mistake:
R v Fotheringham (1989) 88 Cr App R 206 Case summary
R v O'Grady [1987] QB 995  Case summary
R v Hatton [2006] 1 Cr App R 16  Case summary 
If, however, the crime is criminal damage and the defendant is relying on the special defence in S.5(2) Criminal Damage Act, a mistake induced by voluntary intoxication will not bar the application of the defence.
Jaggard v Dickinson [1981] 1 QB 527  Case summary
The defence of mistake