An ambush defence is one in which defence evidence - notably from expert witnesses - has not been adduced in advance to the prosecuting authorities, leading to their inability to rebut it. The term is used in United Kingdom jurisprudence. Since 1987, the possibility of the ambush defence has been much reduced by The Crown Court (Advance Notice of Expert Evidence) Rules 1987, made under section 81 of the Police and Criminal Evidence Act 1984, which in essence require the defence to provide the prosecution with copies of expert witness reports in sufficient time for the prosecution to consider the nature of and if necessary prepare rebuttal evidence opposing the report.