Two insulin specialists leave Letby's scientific camp on the eve of Thirlwall

Helen Shannon, a British chemical engineer, and Geoff Chase, a bioengineering professor in New Zealand, withdrew on 14 September from the scientific effort around Lucy Letby's convictions. They had written a 100-page report last year that questioned the insulin-poisoning findings on two babies and told The Guardian there was a very strong level of reasonable doubt on those two counts. In their new statement they said some arguments now being put forward on Letby's behalf were inconsistent with the available evidence, science and established physiology.

Letby is serving 15 whole-life terms for murdering seven babies and attempting to murder seven others at the Countess of Chester Hospital in 2015 and 2016. A retrial added another attempted murder. The insulin counts were a central plank of the prosecution case because they offered a chemical signature rather than only a statistical cluster of deaths on her shifts. Shannon and Chase attacked that signature. They have not withdrawn that attack. They have drawn a line under other claims in the wider campaign.

The Criminal Cases Review Commission has had a dossier for about 18 months. It is the only body that can send the verdicts back to the Court of Appeal. Its chair has said the commission will read the Thirlwall Inquiry report, due on 15 September. Lady Justice Kathryn Thirlwall was not asked to retry Letby. She was asked how the hospital and the NHS handled alarms about the nurse. She refused to pause her work and called the public fight over guilt noise that added to the families' distress.

The split on 14 September matters because campaign science had been treated in headlines as a single block. It is not. Two people who gave the insulin file its most detailed public critique now say other parts of the innocence brief do not survive basic physiology. That sentence will be in the CCRC's pile beside the Thirlwall chapters on staffing and board delay. It does not free Letby. It does not confirm her. It narrows the scientific dispute to the two insulin babies and leaves the rest of the ward deaths on the jury's findings unless new evidence arrives.

Families who sat through two criminal trials and then through Liverpool Town Hall now watch a third process, the CCRC, and a fourth, the inquiry report, argue over the same nights in 2015. Shannon and Chase have chosen to step off one of those processes rather than lend their names to claims they do not accept. That is a professional decision. It is also a public fact the day before Thirlwall publishes.

Readers should keep the boxes separate. Thirlwall can find that managers were slow and that the unit was weak. The CCRC can still refuse a referral. A referral can still fail in the Court of Appeal. None of those outcomes is decided by a resignation letter from two specialists. What the letter does is strip the campaign of a claim that every scientific name on last year's insulin paper stands behind every later argument. As of 14 September, they do not.