The lead scientific adviser to police in the wake of the Salisbury Novichok poisonings said it is highly likely “many lethal doses” of the nerve agent were applied to Sergei Skripal’s door handle.

The Defence Science and Technology Laboratory (DSTL) witness, known only by the cipher MK26, told the Dawn Sturgess Inquiry that a sample the size of “a third to a sixth of a grain of salt” of the Novichok chemical used in the poisonings could have proved to be a fatal dose for a human.

Former Russian spy Mr Skripal, his daughter Yulia and former police officer Nick Bailey were all poisoned by Novichok in the Wiltshire city in March 2018.

Ms Sturgess, 44, died on July 8 2018 after she was exposed to the nerve agent, which was left in a discarded perfume bottle in nearby Amesbury, Wiltshire.

It is highly likely that there were many lethal doses of the highly toxic specific Novichok applied to the door handle

MK26 said it was “not possible to exclude that there were two bottles” when quizzed on whether they were able to say in an expert capacity that the bottle found in Amesbury was also used for the Salisbury attack.

The witness said: “I think I have tried to express my uncertainty because it’s unclear exactly what the circumstances were of the finding of that bottle and the assembly of that bottle and so without that information, it’s not possible to exclude that there were two bottles.”

Counsel to the inquiry Andrew O’Connor continued: “Just to be clear, the possibility that you are adverting to is the possibility that it was a different bottle, one that has not been found, that was used in Salisbury, and the bottle that we did then find… was a bottle that had been brought along and was discarded without having been used in Salisbury?”

MK26 replied: “I think that’s possible.”

The witness said Russian President Vladimir Putin was “incorrect” to say coming into contact with Novichok would result in someone “dying on the spot” in a diplomatic telegram on March 20 2018.

Mr O’Connor asked: “As to what Vladimir Putin is saying here about someone dying on the spot, in other words within seconds or perhaps minutes, we know of course that neither Sergei Skripal nor Yulia Skripal died… Dawn did die but not on the spot, quite a while later.

“So in far as Mr Putin is saying here that’s incorrect, isn’t it?”

MK26 said: “I would say that’s incorrect and that’s what the evidence shows us.”

On Wednesday, the DSTL witness, based in the Porton Down area of the county, said very few people would have been capable of producing the Novichok used in the attack safely because it is “highly hazardous in small quantities”.

Chairman of the inquiry Lord Hughes made a restriction order, giving anonymity to the witness, which counsel to the inquiry Andrew O’Connor said included their gender.

MK26, who has worked as a research scientists for 20 years, added that the “competence of the scientists working with (the Novichok) would need to be extremely advanced”.

The witness said samples of the nerve agent were taken from Mr Skripal’s door handle 12 days after the poisonings, with rainwater and snow potentially affecting its concentration levels.

They told the inquiry: “After many days of weathering, with the contamination found throughout the locations visited in Salisbury that afternoon by the Skripals, suggests that it is highly likely that there were many lethal doses of the highly toxic specific Novichok applied to the door handle.”

The nerve agent had a concentration level of 97-98%, the witness said.

Speaking about the events of 2018, MK26 said: “We had a lot of staff here (at Porton Down) offering technical support.

“The operations room was running 24/7.

“I haven’t seen this level of activity any time before that or since and I hope I never do again.

“We’re very committed to supporting the safety of the public primarily but also the police with the investigation that followed, and I would also say that it felt very personal.

“It was obviously very close to Porton Down – the majority of staff were living in and around Salisbury or Amesbury when it came to Amesbury and it couldn’t have had any more impact on the people involved than it actually did.

“Obviously, then when Dawn Sturgess sadly passed away, it highlighted to us how dreadful it had been and we were all really sorry that that happened.”

Speaking about how rare the use of a nerve agent is, MK26 said: “I would start from the position that chemical weapons as a whole are very rare and certainly up until the events of 2018, as far as I’m aware, we had never seen the use of Novichok or an appearance of Novichok.

“I would add that there is no reason for these chemicals to be present anywhere other than that they have been made deliberately to cause harm.”

The witness said the poisoning of late Russian opposition leader Alexei Navalny in 2020 was the only other instance in which Novichok or the appearance of the nerve agent had been used, as far as they were aware.

Addressing how difficult it is to produce Novichok, MK26 said: “There are very few that are capable of doing so safely because the chemicals we are talking about are so highly hazardous in such small quantities.”

They continued: “It was clear that whichever laboratory had made this had not only made it but synthesised it which adds a layer of complexity, a layer of sophistication and a layer of hazard to the operators who are undertaking that procedure.”

The inquiry continues.