The collateral use of evidence served in criminal proceedings – is it permissible?

The position on the collateral use of unused material disclosed in criminal proceedings is clear. By virtue of ss.17 & 18 of the Criminal Procedure and Investigations Act 1996, such material may only be deployed in the proceedings in which it was disclosed unless (a) it is required for purposes incidental to the main case (eg an appeal), (b) the material has been referred to in open court or (c) the prior permission of the trial judge has been obtained. Using the material in breach of these exceptions is a contempt of court under s.18 and punishable by imprisonment.

There is however no corresponding statutory provision in relation to used material (ie. evidence) served by the prosecution in criminal cases. In the absence of such a provision, it is sometimes claimed by prosecuting authorities (not least the SFO) that evidence is subject to an implied undertaking from the defendant not to use it for any other purposes. As recently explained by Lavender J. in Global Processing Services (UK) Limited v Yanpolsky [2023] EWHC 425 (KB), however, this is not correct.

In arriving at his conclusion, the judge undertook a careful examination of the authorities on this point. In Mahon v Rahn [1997] 3 WLR 1230, a case concerning events before the introduction of the CPIA 1996, the Court of Appeal ruled that there was no implied undertaking in criminal proceedings in relation to either used or unused material. In Taylor v Director of the SFO [1999] 2 AC 177, also a pre-CPIA case, the Court of Appeal expressed reservations about the correctness of this decision but considered itself bound by it. The House of Lords in Taylor made obiter observations to similar effect. However, although they subsequently overruled Mahon in relation to unused material, they left open the question of whether used material was the subject of any implied undertaking.

Lavender J. concluded therefore that the decision of the Court of Appeal in Mahon represents the current state of the law (ie there is no implied undertaking in relation to used material). It is clear that he had no enthusiasm for this outcome and that, in his view, the collateral use of both evidence and unused material should be forbidden unless one of the relevant exceptions applies. He said as follows:

70. Were the question free from authority, I would see strong arguments, of the kind identified by the Court of Appeal and by Lord Hoffman in Taylor v SFO, for holding that a defendant to criminal proceedings who receives used material from the prosecution is subject to an implied undertaking not to use that material for any collateral purpose. However, the question is not free from authority. On the contrary, Mahon v Rahn is a decision of the Court of Appeal in which it was held that a defendant to criminal proceedings who received used material from the prosecution was not subject to such an implied undertaking. The decision in Mahon v Rahn was overturned by the House of Lords insofar as it related to unused material, but it was not overturned, although it was doubted, by the House of Lords insofar as it related to used material.
71. In those circumstances, I consider that I am bound to follow Mahon v Rahn.

It seems likely that this issue will be revisited in the foreseeable future, possibly in the form of legislation. For now, however, this represents the up to date position. Defence practitioners should take note.