A senior doctor has told a public inquiry it is “likely” that nurse Lucy Letby murdered or assaulted children at the Countess of Chester Hospital’s neonatal unit before June 2015.

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.

Giving evidence to the Thirlwall Inquiry into the events surrounding her crimes, neonatal clinical lead Dr Stephen Brearey said: “On reflection I think it’s likely that Letby didn’t start becoming a killer in June 2015, or didn’t start harming babies in June 2015.

Our perception of what is normal for a neonatal unit in terms of the number of collapses you might expect in a week, a month, or a year, that changed

“I think it’s likely that her actions prior to then over a period of time changed what we perceived to be abnormal.”

Dr Brearey said the reporting culture on the unit was good and staff were very aware to report things they thought had gone wrong, but “in retrospect” he believed some of the incidents between June 2015 and June 2016, and “probably before”, were deteriorations of babies that could have trigged further investigation.

Peter Skelton KC, representing families of Letby’s victims said: “Child A was murdered on June 8 2015. Is it your view that she (Letby) had murdered or assaulted children in your hospital prior to that date?

Dr Brearey said: “I think that’s likely, yes.”

Mr Skelton said: “On reflection now, do you look back and see a number of unexpected collapses or deaths which with information now available to you appear suspicious?”

“Yes,” replied the medic.

Mr Skelton said: “On reflection now, do you recall having any concerns about those collapses or deaths at the time or did your colleagues?”

Dr Brearey said: “No, as far as I’m aware neither me or my colleagues had concerns at the time. We just thought we were going through a busy or particularly difficult patch at times.

“I think events were happening that were unusual. Our perception of what is normal for a neonatal unit in terms of the number of collapses you might expect in a week, a month, or a year, that changed, and I think that was the case for doctors and nursing staff.”

Mr Skelton said: “One of the effects of normalising the abnormal is that when abnormalities occur you don’t recognise it and you don’t take appropriate action – is that a reflection that resonates with you?”

Dr Brearey said: “I think so, yes. We are working in the NHS, it’s not perfect. There are lots of rational ways of trying to explain it away which, at the time, looking back on it, wouldn’t have made sense – blaming the transport service or blaming another trust for maybe babies sending back when they weren’t completely stable, rather than taking a step back and thinking, ‘What’s happening here?’.”

Mr Skelton asked: “Did anyone in your hospital suspect Lucy Letby was harming children prior to June 2015?”

“No,” replied Dr Brearey.

Letby was moved to clerical duties in July 2016 after consultants, including Dr Brearey, told executives about their concerns that she might have been deliberately harming babies, but Cheshire police were not called in until May 2017, and there were plans shortly beforehand to return Letby to the unit.

After three deaths in June 2015, when he was told that Letby was identified as the one member of staff present on each occasion, he recalled his first reaction was to say, “Oh no, not Lucy. Not nice Lucy”.

Counsel to the inquiry Rachel Langdale KC asked: “Why ‘oh no, not nice Lucy’? What was the point of the ‘oh no’, what was the link being made in your mind?”

Dr Brearey said: “Well obviously, some part of my mind was thinking the worst.”

Ms Langdale asked: “What did your mind jump to?”

Dr Brearey said: “The concern that there might be someone harming babies.”

He said the death of a baby girl, Child I, in October 2015 was a “significant moment that raised my level of concern quite considerably”.

The medic said the youngster’s death after a series of collapses – following her return to the Countess after periods of stabilisation when she was transferred to another hospital – had “set a few alarm bells going”.

Ms Langdale said: “By the time of Child I you thought that deliberate harm may be being caused here?”

Dr Brearey said: “Yes, there was considerable concern at that stage.”

The inquiry has previously heard that dislodgement of breathing tubes happened on 40% of shifts that Letby worked when she was a trainee at Liverpool Women’s Hospital, compared with a general occurrence of less than 1% of shifts.

A review by Cheshire Constabulary of the care of 4,000 babies admitted to the Countess of Chester while Letby was working as a neonatal nurse from January 2012 to the end of June 2016 is ongoing.

Police are also investigating Letby’s work placements at Liverpool Women’s Hospital between October and December 2012, and January and February 2015.

The inquiry at Liverpool Town Hall is expected to sit until early 2025, with findings published by late autumn.