It is over 10 years since the Court of Appeal confirmed the legitimacy of the public interest defence in copyright law in Ashdown v Telegraph Group Ltd. However, remarkably little is still known about the defence's function and scope. Indeed, acknowledgement of its unfathomability has become something of a commonplace of copyright jurisprudence and scholarship. This situation contrasts sharply with the position in other related areas of law, such as breach of confidence and defamation, in which courts have made dramatic strides in recasting doctrine to protect ‘public interest’ disclosures. Developments in EU copyright law have also plunged the defence into a renewed crisis of legitimacy. The time is therefore ripe for a reconsideration of the role of the public interest defence in UK copyright law. In this paper, an attempt is made to understand its implicit rationale and scope. It is argued that the existing jurisprudence reflects a persistent concern about copyright's potential to subvert policy outcomes generated by alternative regulatory systems and that the defence is to be viewed as a form of pre-emption doctrine, allowing courts to avoid the explicit rules established under the CDPA in circumstances in which their application would frustrate the outcomes of other more appropriate forms of regulation.