If evidence of wrongdoing is uncovered through improper investigative measures, what protections are there for the subjects of these improper investigations in U.K. law?

Must one still answer for evidence of any wrongdoing no matter how it is obtained or are there any mitigations for this based on investigative impropriety?

  • Edited title… technically I think the fruit of poison tree actually only refers to evidence obtained with the aid of improperly obtained evidence but the real question as explained and elaborated is more fundamentally about the legitimacy of the tree itself rather than merely its fruits.
    – Timothy
    Mar 15, 2022 at 0:20

2 Answers 2


There isn't an automatic exclusion rule for all forms of improperly acquired evidence in the UK. I can't find the exact quote but there was a judgement from a senior court that said in terms: "it's not the court's job to discipline the police but to see that justice is done."

However, judges have the discretion to disallow individual pieces of evidence if they think the interests of justice require it. Section 78 (1) of PACE (1984) has:

In any proceedings the court may refuse to allow evidence on which the prosecution proposes to rely to be given if it appears to the court that, having regard to all the circumstances, including the circumstances in which the evidence was obtained, the admission of the evidence would have such an adverse effect on the fairness of the proceedings that the court ought not to admit it.

Senior judges have been reluctant to lay down general guidelines, holding the trial judge best placed to make these decisions (although of course, subject to appeal). The foundational principle is the accused's right to a fair trial, so in principle one could appeal to the ECtHR under Article 6. However it generally takes a similar line to courts in the UK:

It is not the role of the Court to determine, as a matter of principle, whether particular types of evidence - for example, unlawfully obtained evidence - may be admissible or, indeed, whether the applicant was guilty or not. The question which must be answered is whether the proceedings as a whole, including the way in which the evidence was obtained, were fair.

PACE Section 78(2) is:

Nothing in this section shall prejudice any rule of law requiring a court to exclude evidence.

The general principle in 78 (1) does not override specific prohibitions: for instance, intercept evidence is never admissable under the Regulation of Investigatory Powers Act 2000.

  • +1; note that you can raise ECHR issues in the domestic courts (indeed you must exhaust such possibilities first before going to the ECtHR).
    – JBentley
    Mar 13, 2022 at 20:51

The “fruit of the poisonous tree” doctrine is uniquely US

In all other common law jurisdictions, including the UK, police who behave inappropriately are subject to employment, administrative and judicial sanction as appropriate for their wrongdoing.

Whether evidence gathered inappropriately is admissible is a matter for the particular judge who must weigh the probity of the evidence against the wrongdoing and decide where the interests of justice are best served. It is not automatically admissible nor is it automatically inadmissible.

  • 1
    Where does that come from? Are you not suggesting that the acceptability of the same evidence, gathered in the same way, might be dependant on the wording of the charge, rather than natural justice? Mar 13, 2022 at 22:41
  • 2
    I disagree with the bolded statement. For example in Italy phone recordings cannot be used in court if their recording was not a-priori authorized by a judge
    – Federico
    Mar 14, 2022 at 12:37
  • And note the reason it's unique--it's not a law in the first place. Rather, it's the result of a Supreme Court ruling as there was no consequence for police simply ignoring the rules. The Supreme Court doesn't have the power to decree punishment for the police so it did the only thing it was capable of--toss the improperly-obtained evidence. Unfortunately, the politicians have been content to leave it that way rather than actually pass appropriate laws. Mar 14, 2022 at 23:31
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    Your heading is plainly false. The concept is a fundamental concept in any functioning constitutional democracy. South Africa also has that enshrined in our constitution. It is not unique.
    – Neil Meyer
    Mar 15, 2022 at 15:22
  • 1
    @NeilMeyer South Africa is not a common law country. The “fruit of the poisonous tree” is a common law doctrine unique to the US. Civil law (or mixed law) countries may have similar protections but they are statutory, not common law.
    – Dale M
    Mar 15, 2022 at 20:07

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