Lucy Letby gave a potentially fatal dose of morphine to a newborn baby two years before she murdered her first victim, an inquiry has heard.

The infant received 10 times the correct amount of the painkiller at the end of a night shift in July 2013 and could have died if colleagues at handover had not spotted the error an hour later.

The Thirlwall Inquiry into the events surrounding Letby’s crimes at the Countess of Chester Hospital was told Letby and another nurse had set up the morphine infusion for the youngster.

Letby’s colleague was said to be so “distraught” about the incident that she almost resigned, but Letby was “unhappy” when informed by neonatal unit ward deputy ward manager Yvonne Griffiths that she could not administer controlled drugs until a review had taken place.

A week later, neonatal unit ward manager Eirian Powell apparently informed Letby she could continue working with drugs such as morphine, the inquiry heard.

The next day Letby messaged a colleague, saying: “Thankfully Eirian felt it had been escalated more than it needed to be. Everything is back to how it was.

“I just have to have more training on using the pumps and it will be on my record for six months. She was very supportive, a case of learning to live with it now and getting my confidence back. I’m on nights this week, still feeling a bit vulnerable and thinking ‘what if’ but I’ll get there in time.”

Counsel to the inquiry Nick de la Poer KC asked Ms Griffiths: “Did you escalate it more than it needed to be?”

“No,” said Ms Griffiths.

She said that Ms Powell was on annual leave when she was informed about a “very serious” incident involving morphine.

I just remember the comparison because I know the other lady was very distraught and very upset to the point where she was going to leave nursing. Letby, I think, was upset but not to the same extent

Ms Griffiths said: “If that had not been picked up as soon as it was it might have made the baby demise (sic).”

Mr de la Poer said: “Could have been fatal?”

Ms Griffiths replied: “Yes.”

Asked what Letby’s demeanour was when she first met her about the error, Ms Griffiths said: “I just remember the comparison because I know the other lady was very distraught and very upset to the point where she was going to leave nursing.

“Letby, I think, was upset but not to the same extent. She seemed to accept my decision, I think perhaps she thought I was being a bit harsh.”

Part of the action plan was for Letby to undergo more training on infusion calculations with practice development nurse Yvonne Farmer before she could work with controlled drugs again, but that was not completed until September 2013, the inquiry heard.

Richard Baker KC, representing some of the families of Letby’s victims, said: “One interpretation of what happened is that Lucy Letby went over your head, complained about your decision and you were overruled.

“If that is the conclusion reached, would you with the benefit of hindsight regard that as very manipulative behaviour on the part of Letby?”

Ms Griffiths replied: “I suppose it shows a very overconfident nurse because part of a nursing journey is to learn from any mistakes potentially … I did think that was inappropriate and I agree it was quite manipulative.”

Mr Baker said: “Quite grandiose and arrogant as well, isn’t it?”

Ms Griffith said: “It is, yes.”

Another dosage error by Letby was flagged up in April 2016 when she administered an antibiotic which was not prescribed and not due.

Ms Farmer told the inquiry it was “not a minor error, possibly a major error” but Letby believed it was “unavoidable”.

In her reflections on the incident, Letby wrote: “The mistake was realised immediately by myself and a colleague immediately after the dose had been given and my initial concern was for the safety of the baby. The registrar was informed and measures were taken to ensure that the infant was monitored accordingly and that no harm had occurred.

“Although not excusable at the time myself and my colleague were administering multiple antibiotics all due at a similar time, as well as caring for our own patients and supporting junior members of the team, including a newly qualified nurse, when the unit was not staffed with adequate skill mix.

“On reflection, I feel this situation was unavoidable and care was given to the best of our ability. However, knowing how the circumstances could have potentially ill affected the process of giving antibiotics, I should have been more aware and made an even greater effort to ensure all the relevant checks were made before giving, and time should have been more prioritised more accordingly.”

Letby, 34, from Hereford, is serving 15 whole-life orders after she was convicted at Manchester Crown Court of murdering seven infants and attempting to murder seven others, with two attempts on one of her victims, between June 2015 and June 2016.

The inquiry is expected to sit until early 2025, with findings published by late autumn of that year.