Amel Alghrani and Sarah Bennett

Few criminal cases in recent memory have attracted the level of public attention, legal controversy and emotional intensity as R v Letby [2024] EWCA Crim 748. Yet beyond the headlines lies a subtle but profoundly important legal question: what happens when criminal convictions rest on scientific evidence that is itself uncertain, contested, or insufficiently tested?

High-profile criminal trials are increasingly shaped by complex scientific and medical evidence. In matters such as sudden infant death and shaken baby cases, judicial decisions frequently hinge not on direct witness accounts, but on expert interpretations regarding what happened. Yet in England and Wales, the legal framework governing the admissibility and evaluation of such evidence remains fragmented and has been slow to adapt to the challenges posed by complex scientific evidence. This tension between law and science has rarely been more clearly exposed than in Letby, a case that has come to occupy a contested and deeply unsettling place in the modern criminal justice landscape.

Lucy Letby, a neonatal nurse at the Countess of Chester Hospital, was convicted of the murder of seven infants and the attempted murder of seven others between 2015 and 2016. The profound suffering experienced by the families involved must not be obscured by the legal analysis that follows.

At Letby’s trial, in the absence of direct eyewitness evidence, expert medical and statistical testimony played a central role in providing the framework through which the jury was invited to interpret a series of unexplained clinical collapses and deaths. This is not to suggest that the prosecution case rested exclusively on expert testimony; it also relied on circumstantial evidence including Letby’s presence during relevant incidents, documentary material and wider behavioural evidence. However, expert testimony provided an important interpretive framework, shaping how much of this material was understood and evaluated. That is what makes the case so legally significant. It raises fundamental questions about how criminal courts assess scientific reliability, how effectively juries can evaluate technical evidence and whether existing safeguards are sufficient where expert opinion carries extraordinary weight.

Although the Court of Appeal upheld both the admissibility of the expert evidence and the adequacy of the trial judge’s directions, the controversy surrounding the case has persisted. Questions remain relating to the reliability of the underlying medical evidence and the extent to which the adversarial process is capable of rigorously testing complex scientific claims. Significantly, many of the most prominent criticisms of the prosecution evidence did not emerge through competing defence experts at trial. Rather, they surfaced post-conviction, as independent clinicians, statisticians and commentators began scrutinising the evidential foundations of the case.

The argument advanced here is not that the Court of Appeal erred, nor that Letby’s convictions are unsafe. Such determinations remain matters for the appellate process and any future review. Nor is it to deny that the prosecution case relied on wider circumstantial evidence beyond expert testimony. Rather, in a criminal law context, the significance of the Letby case lies in what it reveals about the limits of existing safeguards when expert evidence plays a central role in shaping the meaning of wider facts. Whilst Letby’s defence team subjected prosecution experts to cross-examination, ultimately the defence did not call independent medical experts to give evidence at trial. Consequently, the jury was asked to evaluate complex medical evidence without hearing competing expert accounts of what may have caused the clinical events in question. This does not, in itself constitute proof of systemic failure. Defence decisions may reflect strategy, resources, or difficulties identifying appropriate experts. Nonetheless, it does expose the fragility of a system that continues to treat adversarial challenge as its primary safeguard of reliability.

The difficulty exposed by R v Letby is therefore not simply a matter of doctrinal fine-tuning. It points to broader structural challenges in how criminal courts engage with contested scientific evidence. Existing legal safeguards are not always well equipped to evaluate complex questions of reliability in a principled, consistent and transparent manner. Judges are often required to rule on reliability without the benefit of a clear statutory reliability framework. Juries may be asked to assess highly technical claims with limited assistance in understanding scientific uncertainty and methodology. Procedural safeguards exist, but they remain inconsistently applied and are not always well suited to the demands of complex scientific evidence.

Although the common law and procedural rules provide a formal framework, they do not always adequately address the epistemic challenges posed by modern scientific evidence. Legislative inertia following the Law Commission’s 2011 recommendations on expert evidence has left England and Wales with a fragmented regime, the effectiveness of which depends heavily on judicial confidence, access to appropriate expertise and strategic choices made during litigation. Continued reliance on adversarial challenge and broad judicial discretion as the central safeguards of reliability reflects a procedural model developed for a very different evidential landscape, one that sits uneasily with the complexity of modern scientific evidence.

The ongoing scrutiny of the Letby case, including a reported application to the Criminal Cases Review Commission, alongside the anticipated publication of the Thirlwall Inquiry findings ensures that these questions remain live.

Our article argues that the lessons of Letby extend far beyond a single controversial prosecution. If courts are to maintain public confidence in cases involving complex scientific evidence, more robust mechanisms for evaluating how expert knowledge is produced, challenged and limited are needed. A legal system increasingly asked to adjudicate scientific disputes must be better equipped to engage with methodological uncertainty. The stakes in such cases could scarcely be higher. These issues are explored in greater detail in our recent article, A Alghrani and S Bennett ‘Expert evidence in criminal law: rethinking reliability in England and Wales after the Lucy Letby Case’, now published in the ADVANCE section of the Northern Ireland Legal Quarterly.