A doctor who exchanged more than 1,000 “inappropriate” and “frivolous” Facebook messages with Lucy Letby secured her a placement at another hospital while she was suspected of murdering babies, a public inquiry has heard.

The registrar, who can only be named as Dr U, said he had supported the nurse because she was “struggling with her mental health” but that he now believed he had been “misled and maybe manipulated”.

Giving evidence to the Thirlwall inquiry on Monday, Dr U said he “wasn’t aware of the full clinical picture” when he began messaging the nurse in June 2016 – a year after she murdered her first newborn victim at the Countess of Chester hospital.

By this time senior doctors openly suspected that Letby, now 34, was harming babies and had demanded that she be removed from the hospital’s neonatal unit.

Rachel Langdale KC, counsel to the inquiry, said Dr U had exchanged 1,355 Facebook messages with Letby from June 2016, spanning from “quite frivolous casual conversation” to “entirely inappropriate” discussions about newborn patients.

Asked to explain the messages, the registrar said: “Letby was struggling with her mental health and I think I picked up on that and I offered some support and that support, it grew, and I understand that she slept very poorly because of worry and anxiety.

“There were often messages that were passed throughout the day and sometimes late at night [and] earlier in the morning.”

In one message after Letby was removed from the neonatal unit in July 2016, Dr U told her: “You’re still the best [neonatal unit] nurse I’ve ever worked with.”

In another he wrote: “If anyone says anything to you about not being good enough or performing adequately I want you to promise me that you give me details to provide a statement.”

The public inquiry was told that Dr U provided Letby with confidential information about the efforts to investigate the cause of unusual and unexpected deaths on the neonatal unit after she was removed from it in July 2016.

Dr U said he wanted to “reassure her” at the time but that he now believed this was a “massive mistake”.

He said “in hindsight” he did not believe the messages were appropriate and that the parents of the babies involved would find them “very upsetting”.

“I’ve reflected on this daily since – and certainly since my first police interviews in January 2018 – and I think I’ve become more aware that I wasn’t aware of the full clinical picture and I provided support by being misled and maybe manipulated,” he said.

“And for that I am really sorry that things have come to end as they have. I have a lot of regrets about how that period of time took place.”

The Thirlwall inquiry was also told that Dr U had secured Letby a placement at another hospital in January 2017 – six months after she had been moved from frontline nursing to a clerical role due to concerns about her harming babies.

The inquiry heard a statement on behalf of the hospital – which cannot be identified for legal reasons – confirming that Letby attended on various dates between January and April 2017 under Dr U’s supervision.

In the statement, a hospital executive said Dr U’s request on Letby’s behalf was “informal in nature” and that senior managers had been unaware, “therefore no inquiries had been made about her role or background”.

During her first criminal trial last year, Letby denied that Dr U was her boyfriend and said he was someone she “loved as a friend”. She broke down in tears when he gave evidence for the prosecution at Manchester crown court.

The inquiry was told last month that the mother of one of Letby’s victims, Child N, had submitted a formal complaint about Dr U for discussing her infant son’s condition on Facebook. She said the doctor showed a “disregard for blatant breaches of patient confidentiality”.

The mother said she lodged the complaint last year with PALS (Patient Advice and Liaison Service) at the hospital where this doctor now works. She has yet to be told of any outcome, the inquiry heard.

Letby is serving a whole-life prison term after being found guilty of murdering seven babies and attempting to murder another seven between June 2015 and June 2016.