Procedural rules prescribe the ways in which law enforcement agencies can obtain evidence that can be used in a subsequent prosecution. Inevitably, these rules are sometimes broken. Sometimes, though, the evidence found remains highly probative and potentially indicative of guilt. This presents an evident dilemma. If establishing truth is paramount, excluding relevant evidence is counterproductive. Worse, it would appear that such a stance would increase the potential for a wrongful conclusion to be drawn, which could result in a miscarriage of justice, even though this is more likely to be an undeserved acquittal than a suspect conviction. However, all rules of procedure have a purpose. They have been drafted so as to ensure (at least theoretically) that individuals are not put in situations where they have to provide samples, be searched or allow law enforcement officers to search their property without good cause. A lax approach when the rules are breached would provide little incentive for compliance on the part of law enforcement personnel. The issue may not appear so pressing if, say, an illegal search reveals that the suspect was handling stolen goods. But a failure to provide an effective remedy also has the potential to facilitate (as opposed to legitimate) unlawful searches that yield nothing. How such evidence is treated poses a challenge: there is an obvious temptation to admit that which might aid a deserved finding of guilt, but with a reservation that, in so doing, the state would benefit from breaking the law.

This chapter will start with an account of the English common law position, which generally holds that illegally, improperly or unfairly obtained evidence remains admissible at trial, save in exceptional cases. Section 78 of the Police and Criminal Evidence Act 1984 now governs the admissibility of illegally or unfairly obtained evidence and this forms the focus of the next section. The Police and Criminal Evidence Act 1984 departs from the common law in that courts must consider the circumstances in which evidence was obtained when determining whether admitting the evidence would render the proceedings unfair. A recent appeal case, R v Twigg [2019] EWCA Crim 1553, will be used to illustrate how a blatant procedural breach (though taken in good faith), which yielded highly incriminating evidence, was dealt with by the Court of Appeal.