A widower who was sent a whistleblowing letter alleging surgical errors before his wife’s death has expressed surprise that the hospital involved demanded that staff provide fingerprints to identify the anonymous sender.

Jon Warby, a former police officer, said the letter “knocked me sideways” when it arrived two months after his wife, Susan, had died aged 57 after an operation for a perforated bowel at West Suffolk hospital (WSH) in 2018.

Speaking after the adjournment of an inquest in Ipswich into Susan’s death, Warby confirmed that the letter outlined errors in his wife’s surgery that had already been disclosed to him by the hospital. But he also revealed that it contained allegations about the treatment of at least one other patient. He declined to give details.

Warby said the allegations concerned “a previous incident which is not being looked at here and we can’t comment on”. He could not confirm whether the allegations involved the death of another patient at WSH – the local hospital of the health secretary, Matt Hancock.

Last month the Guardian revealed that the anonymous letter had prompted an unprecedented “witch-hunt” by the hospital bosses to try to identify the staff member who had sent the letter. This week the Guardian further revealed that the trust had spent more than £2,500 on handwriting and fingerprint experts after demanding samples from staff.

Asked by reporters whether he was upset by such tactics, Warby said: “I was quite surprised the lengths they went to, but that was also part of a criminal investigation as well. So I can understand that aspect of it to a degree, because the allegations involved in the letter could potentially involve a criminal case.”

In a written statement issued after the adjournment, Warby added: “Transparency is important in maintaining confidence in the NHS and upholding patient safety, and hospital staff should feel able to speak out on any issues they identify in the workplace.”

Warby said he was satisfied that the inquest had addressed all allegations in the letter specifically relating to his wife’s treatment.

The inquest heard that during one of Mrs Warby’s operations the wrong fluid had been used in an IV drip. Instead of saline, a solution containing glucose was injected into Warby’s artery and as a result her blood sugar increased to very high levels. Doctors did not realise the mistake and then treated her with insulin to bring the blood glucose levels down, despite the risk this carried of brain damage. The inquest also heard that Warby’s lung had been punctured during a procedure to insert a central line into her inner jugular vein.

Frances McClenaghan, a lawyer for WSH, told the inquest: “The trust accepted at a very early stage that the incorrect fluids had been administered.”

She described the punctured lung incident as a “recognised complication” of the operation.

Before the inquest was adjourned Warby’s lawyer, Conor Dufficy, said the family accepted that these errors had not contributed to her death. But he asked the coroner to seek expertise from an independent witness, because all the witnesses called to the inquest so far worked, or had worked, at WSH.

The coroner agreed to the request and adjourned the hearing.

Warby said he welcomed the decision. In his statement he said: “Sadly, questions still remain about whether Sue could have survived if these errors had not been made, and I will continue to seek answers in relation to this.”

He added: “The one positive to come out of all of this is the changes that have been put in place to protect future patients and prevent future incidents. Nobody should have to go through what we have been through.”

Senior doctors at West Suffolk hospital said the trust’s demand for fingerprints and handwriting typified a bullying management style. They were also angered when Hancock failed to respond to concerns about the trust’s treatment of staff.

Dr Rinesh Parmar, chair of the Doctors’ Association UK, said: “The witch-hunt for a whistleblower following the tragic death of Mrs Warby highlights a deep-seated toxic culture at West Suffolk hospital. A culture where alleged bullying and intimidation of senior doctors is the chosen strategy by senior managers to silence whistleblowers concerned about patient safety.”

He added: “Rather than address the widespread patient safety issues leading to Mrs Warby’s death, the senior management at the trust sought instead to divert their attention to targeting the whistleblower. Meanwhile the health and social care secretary, Matt Hancock, who purports to champion patient safety, and in whose local hospital this is happening, has simply buried his head in the sand when approached about the matter.

“The very fact that a clinician felt it was necessary to contact the family directly in an anonymous manner is all too telling of a draconian blame culture where staff feel too afraid to speak up when things go wrong. There is no doubt that staff’s worst fears have been confirmed by the trust’s subsequent punitive and needlessly high-handed response.”