What the Cannings principle is
The 2003 Court of Appeal judgment in R v Cannings addressed a series of cases in which mothers had been convicted of infanticide on retrospective re-interpretation of previously-explained deaths by paediatric experts. The conviction in Cannings was quashed. The judgment articulated a principle that has since been applied in other medical-evidence miscarriage-of-justice cases.
The principle, in its operative form: where a conviction depends on medical expert evidence, and reputable medical experts disagree about the cause of the death or injury, the conviction is unsafe. This is not a blanket rule against prosecuting on medical evidence; it is a rule against prosecuting where the medical evidence is itself in genuine dispute among professionals.
Why the principle applies to Letby
The current state of the Letby medical evidence is precisely the state the Cannings principle addresses. Reputable medical experts — fourteen senior international neonatologists on the Shoo Lee Panel, the Joint Insulin Report authors, the independent paediatric pathologists who have reviewed the post-mortem material — disagree with the Crown’s causation experts on every indicted case.
This is not speculative disagreement. It is sustained, peer-reviewed, institutionally-supported disagreement across eight countries and multiple sub-specialisms. On the face of the Cannings principle, the Letby convictions are unsafe.
The three post-Meadow cases together
Sally Clark (acquitted 2003), Angela Cannings (acquitted 2003), and Donna Anthony (acquitted 2005) together form the post-Meadow miscarriage-of-justice sequence. In each case, the conviction depended on expert testimony from Sir Roy Meadow or related paediatric-pathology experts. In each case, exoneration followed disagreement from other reputable medical specialists.
The Royal Statistical Society’s post-Clark framework, the Cannings principle, and the Anthony quashing together form the legal-medical framework the CCRC is now applying to the Letby case.
What the Cannings principle does not say
The Cannings principle is narrower than a claim that medical-evidence cases should not be prosecuted. It applies where the medical evidence is the principal basis for conviction and where reputable experts disagree. On indictments supported by strong physical-exhibit evidence or direct eyewitness evidence, the principle does not apply in the same way.
In the Letby case, the medical evidence is the principal basis for conviction. The shift-rota chart, the notes, the Facebook searches, the handover sheets — each was presented to the jury as corroboration of the medical causation evidence, not as independent proof. If the medical causation evidence falls, the corroborating strands fall with it. The Cannings principle therefore applies to the Letby case in its strongest form.
The CCRC application
The October 2025 CCRC application filed by Mark McDonald KC relies on the Cannings principle as one ground for referral. The thirty-one-plus independent expert reports accompanying the application are the evidential basis for satisfying it: reputable medical experts, on multiple specialisms, disagree with the Crown’s causation opinion.