Salisbury District Hospital staff on duty the day Yulia and Sergei Skripal were poisoned in March have said once the nerve agent was detected “all the evidence was there that they would not survive”.

Speaking exclusively to BBC Newsnight, Sister Sarah Clark, told how initially the case appeared more or less routine.

She said: “We were just told that there were two patients down in the emergency department who were critically unwell and they would be coming up to the unit.”

But after police identified the victims as Yulia Skripal and her father, one-time Russian spy, Sergei, it became apparent it may not be an ordinary case.

Dr Stephen Jukes, the Intensive Care Consultant who treated the Skripals a week after they arrived at the hospital, said on admission the medical team suspected both were suffering an opioid overdose but the diagnosis quickly changed to a nerve agent poisoning.

BBC Newsnight was told that at first it was simply a matter of keeping the Skripals alive.

They were heavily sedated and given heavy doses of drugs designed to protect them from the effects of the poison, identified as Novichok, to help re-start their bodies’ natural production of a key enzyme.

But it wasn’t until when PC Nick Bailey also fell ill that staff realised they too may also be at risk.

Lorna Wilkinson, the Director of Nursing at the hospital spoke of the concern she felt for the potential scale of the incident [when] the policeman was admitted, shortly after the Skripals, and exhibiting similar symptoms.

She said: “I suppose the key marker for me was when the PC [Nick Bailey] was admitted with symptoms – there was a real concern as to how big could this get.”

She recalled thinking: ”‘Have we just gone from having two index patients [to] having something that actually could become all-consuming and involve many casualties?’ because we really didn’t know at that point.”

Dr Duncan Murray, Head of Intensive Care Department, summed up how surreal the events were, saying: “I spoke to the nurse in charge [who] had been on that night and it was this conversation I really could never have imagined in my wildest imagination having with anyone.”

Both Skripals have since been discharged from hospital.

When asked for the long-term prognosis for the Skripals and PC Bailey, Dr Christine Blanshard, Medical Director at the hospital, told BBC Newsnight, “the honest answer is we don’t know”.

Much of the uncertainty and confusion surrounding Novichok poisoning stems from the simple fact it has never happened before.

Blanshard said that the “total world experience” of treating people who have been poisoned with Novichok is that which was administered to the three admitted to Salisbury District Hospital in early March and that “it’s safe to say we’re still learning”.

She said she was uncertain if they will be in need of long-term medical assistance and care.

Dr Duncan Murray praised the “excellent teamwork by the doctors, fantastic care and dedication by our nurses” in the treatment of the Skripals and PC Nick Bailey.

He said that “international experts”, some of them from the Porton Down laboratory, internationally known for its chemical weapons expertise, assisted by helping process tests and offering advice on the best therapies.

Yulia was discharged in April – and last week made a statement thanking the hospital – but it took several more weeks before Sergei Skripal could follow her out of the hospital. They’ve been brought back from the brink of death, but questions remain about the longer term.