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In criminal trials, the defendant does not always tell the truth. There is, in general, no objection to the prosecutions' introducing evidence that shows that something the defendant said was untrue. For example, if the defendant claimed to be elsewhere at the time of the offence, the prosecution will normally be allowed to call a witness to testify that he saw the defendant at the scene of the crime.

A more difficult problem is to what extent evidence is admissible to show a general propensity to be untruthfull. Such evidence is a species of EvidenceOfBadCharacter, and at common law was rarely admissible.

Under the Criminal Justice Act (2003)(still in force at the time of writing), evidence of the defendant's bad character, including untruthfullness, could be adduced in cross-examination, (if the defendant testified) if the defendant put his own good character in issue (see putting character in issue), or if he testified against a co-defendant, or if he made an attack on another person's character.

The scope for admissibility of evidence showing a propensity to be untruthfull will be substantially increased by the Criminal Justice Act (2003). s.103(1)(b) admits evidence that goes to the question

whether the defendant has a propensity to be untruthful, except where it is not suggested that the defendant's case is untruthful in any respect

There is a presumption in favour of such evidence being admitted, with a discretion to exclude it under 101(3) if it would have an adverse effect on proceedings.