Evidence used in the trial of Lucy Letby showing which staff came in and out of a baby unit was incorrect, admits the Crown Prosecution Service.

Last year ‘killer nurse’ Lucy Letby was found guilty of the murder of seven babies and the attempted murder of six others at the Countess of Chester Hospital.

In July, a jury at a retrial convicted Letby of attempting to murder a seventh child – known as Baby K.

But prosecutor Nick Johnson told the court during her retrial that data showing which nurses and doctors swiped in to and out of the intensive care ward had been ‘mislabelled’.

Although the discrepancy was discovered and corrected in time for the retrial, Conservative MP Sir David Davis has written to chief crown prosecutor of Mersey-Cheshire CPS Sarah Hammond demanding clarification.

Sir David 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.’

In both trials, door swipe data was used to show that Letby was the ‘common denominator’ and the only nurse present in the unit when babies rapidly deteriorated.

Prosecutors in the initial trial last August used the data to show that Letby was the only nurse on the ward when Baby K’s breathing tube was dislodged at 3.47am.

But in the retrial the door swipe data was amended to show that the baby’s designated nurse had returned to the intensive care unit and that Letby was, in fact, not alone.

During the initial trial, the prosecution argued that Letby was ‘a constant malevolent presence’ when babies became ill or died at the Countess of Chester Hospital.

They used door swipe data to show that Letby was the only nurse present when Baby O first collapsed.

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’.

In a separate incident, Baby A’s collapse was found to be consistent with a deliberate injection of air when only Letby was in the room.

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

Letby received a whole life sentence and will die in prison as a result of her crimes. 

The validity of Letby’s initial trial has come under scrutiny in recent months as several scientists second-guessed the medical evidence heard by the jury.

Sir Davis is currently analysing the evidence used to convict the neonatal nurse.

The former Brexit secretary launched his investigation after doubts were raised about Letby’s guilt – with some members of the Royal Statistical Society expressing concerns over the use of statistics to secure a conviction on the basis of probabilities.

Mr Davis, a civil liberties campaigner, is understood to be concerned about the justice system’s institutional reluctance to admit to its own failings, leading innocent people to languish in prison.

Last week he told The Daily T Podcast that he would visit Letby in prison if he was ‘at least three-quarters persuaded that she’s innocent’.

He said: ‘I may not be allowed to, her lawyer may say no; she may say no. That’s got to be with her permission.

‘For me to do anything about this, I’ve got to be at least three-quarters persuaded that she’s innocent.

‘I’ll read millions of words of evidence over August and come to a conclusion in probably September or maybe October.’

Mr Davis has said that if after speaking to experts, he does come to the conclusion that she is innocent he will raise the case with parliament and seek to get a Criminal Case Review Commission referral.