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Traditionally UK courts have been very reluctant to allow a person to claim that his ignorance of the law is a defence to a criminal charge. This is true where even where it would have been almost impossible for the defendant to know the law (e.g., it changed while he was out of the country).

An partial exception may exist where it can be shown that the defendant's ignorance obviates the Mens Rea of the offence. For example, if the offence requires that a person intend to appropriate property belonging to another, and he takes property which he thinks has been legally disclaimed by its former owner, then he may not have the Mens Rea of the offence. In this case, the defence is not ignorance of the law, but mistake of law obviating Mens Rea. The distinction is a fine one, and it is not obvious that the defence has frequently succeeded. However, a statute may specifically allow a defence of this sort in certain cases. For example, it is a defence to a charge of Theft for the defendant to show that he honestly believed he had a right to take the property.