By Jonny Humphries & Angela Ferguson

It was discussed at a high-level meeting which concluded that "no further investigation was warranted".

Ms Langdale said: “With hindsight, this decision may represent a significant opportunity missed."

By February 2016, consultants on the unit, including Dr Stephen Brearey, had begun to question Letby's presence at a number of unexpected and unexplained incidents.

Ms Langdale says a meeting took place on 8 February, 2016, involving Dr Brearey, his colleague Dr Ravi Jayaram, and other senior hospital figures to discuss and review the deaths of babies where the diagnoses had been uncertain.

The inquiry heard Dr Brearey recalled raising the fact that six of the nine babies had collapsed between 00:00 and 04:00, and that a particular member of staff - Letby - had been present.

In June Dr Brearey emailed hospital executives to say that he and his colleagues "felt that on the basis of ensuring patient safety on [the neonatal unit] this member of staff [Letby] should not have any further patient contact on [the unit]".

The inquiry heard that ward manager Eirian Powell moved Letby from night shifts on to day shifts on 7 April 2016 in a move she later described as aiming to "make sure that [Letby] was alright" and that she was watched.

Ms Langdale told the inquiry: "The decision to move Letby to day shifts raises serious questions which we will be investigating.

"If there was sufficient concern to take Letby off night shifts, then how could a decision that left Letby in sole charge of neonatal babies during the day be justified?"

It was not until July that year following further deaths that Letby was moved off the ward, into a backroom team without patient contact.

However, this was seen by hospital managers as a "temporary measure" and there were discussions about moving Letby back on to the ward in January 2017 - which was prevented due to staffing pressures meaning she could not be supervised.

But Letby was told she would be moved onto the ward again in April 2017, which led to a backlash from senior consultants.

Ms Langdale said: "Her planned return to the ward on 3 April 2017 only appears to have been stopped because of the tenacious lobbying of the consultants.

"But for their determined approach, it appears likely that she would have been permitted to return to dealing with babies."

Ms Langdale said: "History tells us that serial killers are deceptive, manipulative and skilled at hiding in plain sight."

She said an inquiry into Harold Shipman, a GP thought to have murdered hundreds of his patients, shed little light on why he carried out his crimes and found he was able to kill undetected over many years.

She added: "For ordinary, decent right-thinking people the actions of Letby will remain unfathomable. We will not be inviting speculation from witnesses about her motive or mindset."

'Hiding in plain sight'

Earlier Lady Justice Thirlwall told the hearing that since the Court of Appeal judgement rejecting Letby’s appeal, there had been a "huge outpouring of comment" from some quarters about the validity of her convictions.

She said that "noise" had caused "enormous additional distress" to the parents of those babies.

The inquiry later heard Letby made a number of visits to Alder Hey Children's Hospital after she was removed from nursing duties at the Countess of Chester.

The former nurse was also said to have visited the Countess's neonatal unit in the same period, including a "tea party" to welcome her expected return to the ward.

Ms Langdale said the inquiry would hear evidence that Letby went on to make a number of supervised visits to Alder Hey including outpatient, clinics, ward rounds and team meetings - although a witness would say they believed she had no known unsupervised patient contact.

The inquiry is expected to last until at least December.