Evidence presented in the case of Lucy Letby showing which staff came in and out of the baby unit was wrong, the Crown Prosecution Service has admitted.

Last August, Letby was convicted of the murders of seven babies and the attempted murders of six others at the Countess of Chester Hospital.

A retrial earlier this year found her guilty of the attempted murder of another child, known as Baby K.

During the retrial, Nick Johnson KC, prosecuting, told the court that door swipe data, showing which nurses and doctors were entering and exiting the intensive care ward, had been “mislabelled”.

The Crown Prosecution Service told The Telegraph that the discrepancy discovered was related to one door in the neonatal intensive care unit, and that it had been corrected for the retrial.

Now Sir David Davis, the Tory MP, has written to Sarah Hammond, the chief crown prosecutor of Mersey-Cheshire CPS, asking her to “urgently make clear” what timing errors were made during the first trial and how they relate to the prosecution arguments.

In several cases in the first trial, the door swipe data was used to show that Letby was the only nurse present in the unit when babies suffered catastrophic collapses. But the CPS refused to confirm that all other door swipe data was correct during that first trial.

The trial of Letby has come under scrutiny in recent months, with several scientists and doctors questioning the statistical and medical evidence that was put before the jury.

There are concerns that not enough weight was given to levels of understaffing, poor practice and cramped conditions in the baby unit, as well as the severe prematurity of the babies.

The Royal Statistical Society is due to convene a meeting in the wake of the verdicts, stating that it was aware of “concerns” from its members and the wider community regarding the use of statistical evidence in the case.

Since the trial, it has also emerged that the ward struggled to contain infectious outbreaks that may have put the babies at risk.

Sir David, who is analysing the evidence used to convict the neonatal nurse and is planning to bring a parliamentary debate after the summer recess, said: “The door swipe data is clearly vital to knowing which nurse was where at one point in time, and this in turn was vital to the prosecution’s case in the first trial.

“It is therefore essential that the CPS makes it plain whether those errors occurred throughout any of the evidence of the first trial.”

Letby received a whole life sentence, meaning she will die in prison. After her conviction, Judge Goss said she had led a “cruel, calculating and cynical campaign of child murder”, while the police said she had abused the trust placed in her in the most unthinkable way.

The families of the victims have spoken of their anguish that anyone would question the verdict or show sympathy to Letby.

In the initial trial, the prosecution said Dr Ravi Jayaram, a consultant, had discovered Letby standing over Baby K at 3.50am on Feb 17, 2016. The baby was deteriorating, and its breathing tube had been dislodged.

The prosecution said door swipe data showed the baby’s designated nurse had left the intensive care unit at 3.47am. But the data was amended in the retrial to show the nurse had returned at that time, meaning Letby was not alone.

During the retrial, both the prosecution and the defence accepted that it was a genuine mistake, and Letby was convicted of the attempted murder of Baby K. 

But it is unclear whether the error could also have had a significant impact on how evidence was presented to the jury in the initial trial.

Throughout the first trial, swipe data was used to show Letby moving around the neonatal unit, and in the prosecution opening Mr Johnson described the nurse as “the common denominator” and “a constant malevolent presence” when babies became ill or died.

In the case of the murder of Baby O from an injection of air, door swipe data was used to show that Letby had entered the unit at 2.39pm on June 23 2016.

The prosecution said that “within a few minutes of Lucy Letby coming back on the neonatal unit, as shown by the door entry swipe system, [the baby] suffered his first collapse”. It was also used to show another nurse did not come back until after the collapse.

In the case of Baby A, a prosecution medical expert told the court that the baby’s collapse was consistent with a deliberate injection of air a minute or two before deterioration, when only Letby was present.

Letby was also said to be the only nurse present for the attempted murder of Baby N at 1.05am on June 3, 2016, after a nurse took a break and left the unit at 1am.

In the case of the murder of Baby D, she was accused of being alone with the infant, although she told police she could not recall that being the case.

Mr Johnson told the court: “As with all these cases – it is the coincidence of problems happening when Lucy Letby was about and the coincidence of the same problems happening with different babies at different times which is so telling and indicates that it was her malign hand at work.”

A spokesman for the Mersey-Cheshire Crown Prosecution Service said: “The CPS can confirm that accurate door swipe data was presented in the retrial.”

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