R v King  1 QB 285
Mistake on the facts as a defence to a charge of bigamy under Offences Against the Person Act 1861
The defendant had remarried while his previous marriage was still valid. He was prosecuted for bigamy contrary to section 57 of the Offences Against the Person Act 1861, which states that:
“Whosoever, being married, shall marry any other person during the life of the former husband or wife, whether the second marriage shall have taken place in England or Ireland or elsewhere, shall be guilty of felony…” (OAPA 1861, s.57).
His argument had been that he had entered into the new marriage under the honest mistaken belief that his previous marriage was void.
Whether a genuine mistaken belief as to a fact is a defence to a charge of bigamy under s.57 OAPA 1861.
Mistake can be used successfully as a defence in this context only where the belief is not only honest, but also reasonable – this would negate the mens rea requirement of s.57 OAPA 1861. The court distinguished R v. Wheat & Stocks 2 K.B 119where even a reasonable belief was not held to be sufficient for a defence (that case was in any event overruled byR v. Gould  2 Q.B 65 which further entrenched the rule that honest and reasonable belief could amount to a defence to a charge of bigamy). An unreasonable belief does not suffice, inter alia, because it betrays negligence as to the facts on which the belief is based.