One of the Russian chemical weapons scientists who developed the Novichok nerve agent has warned that hundreds of people could be at risk for years following the attack in Salisbury.

Vil Mirzayanov, who fled to the US two decades ago, claimed Sergei Skripal and his daughter would not recover from the poisoning.

“There is no cure,” he told Sky News from his home in New Jersey. “There are antidotes but…they will be invalid for whole life.”

Dr Mirzayanov said Novichok was so powerful that extremely small doses could remain a danger to public health for years, listing possible symptoms including headaches and loss of coordination.

“It’s very bad because even the very small doses, very small, still they are very effective and then there will be consequences for years probably,” he added.

The former Soviet Union scientist said public health advice, including washing clothes and sealing belongings was “not enough” and confirmed that hundreds of people could be at risk.

Asked whether he felt guilty for his part in developing Novichok, he added: “I participated in this criminal enterprise, because of that I’m probably the most fiery enemy of these chemical weapons.

“It’s a weapon of mass murder.”

The warning came as the Government sought to reassure the public that the wider risk from the attack on 4 March remained low.

Neil Basu provides Scotland Yard statement on Salisbury incident

Public Health England (PHE) issued advice for people who visited The Mill pub and Zizzi restaurant, where Mr Skripal and his daughter ate and drank before falling ill, to wash their clothes and belongings, and seal off anything that cannot be manually cleaned.

Neil Basu, Assistant Commissioner of the Metropolitan Police, said 38 people had been seen by doctors in relation to the incident.

He told a press conference that 34 of those patients had been assessed and discharged, with Mr Skripal, his daughter and Detective Sergeant Nick Bailey remaining in hospital and one person being monitored as an outpatient.

The officer said anyone known to have been in the same area as the victims has been directly contacted, adding: “This is over a week later – people are not presenting symptoms, we are not seeing them.”

Professor Paul Cosford, medical director and director of health protection at PHE said there was "no evidence to support the suggestion that short-term exposure to trace quantities" would affect people's health.

“The health effects of chemical exposures are generally related to the dose received," he added. "It is clear that this substance can have a serious immediate effect on those who have a significant dose.

"In contrast, the general public who were in the Mill Pub or Zizzi’s restaurant could only have been exposed to extremely small traces, if any. There will have been a huge difference between the dose those requiring treatment received, and any received by the public."

Life continued in Salisbury’s city centre, where shoppers have said they are getting on more or less as usual around police cordons.

Mid-way through the afternoon key shopping streets remained busy and, were it not for a circling helicopter and a few extra police officers, a visitor might not realise anything was amiss.

Kate Chivers, 22, did not believe Dr Mirzayanov’s scenario was “100 per cent that it will happen”.

“If it was that bad the police would have evacuated,” she told The Independent.” Everyone within the ring road would be asked to leave.”

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But, she admitted, “it freaks me out”.

A local cleaner, who asked not to be named, said: “It's very scary. The whole thing is quite frightening."”I'm in town a lot, I work in town. If it is that what we're talking about is minute, pin-prick amounts and we wouldn't know immediately, that's scary.

"Particularly if you don't get symptoms in the here and now. And would you for a minute think, 'that's because...', you might just think you're unwell for other reasons."

She added: "What's worrying me even more than that is what's this going to do with our relationship with Russia? That's kind of scary. I hate that feeling that this has put us in conflict with Russia."

Gareth Hancock, a 17-year-old housekeeper at Wetherspoons opposite The Mill pub, said: ”I think it's mad that it's actually been used around innocent people who've done nothing wrong.

"I was quite close to where he was on the Sunday. I walked just through The Maltings on my break to get some food.

"It's quite a shock to think that you can come into contact with it that easily without knowing about it."

Mr Basu said he could not comment on how the nerve agent was administered but vowed that police would leave “no stone unturned in establishing the full circumstances of the attack”

He issued a new appeal for anyone who saw the pair in Mr Skripal’s car – a red BMW – in Salisbury between 1pm and 1.45pm on the day of the attack to call police.

More than 250 police officers are currently working on the investigation, assessing 380 pieces of evidence and hours of CCTV footage.

They have established that Ms Skripal had arrived at Heathrow Airport to visit her father on 3 March, and they arrived in the Sainsbury’s upper level car park in The Maltings at 1.40pm the following day.

Theresa May: 'highly likely' Russia was responsible for poisoning of Sergei Skripal

They went to The Mill pub before going to Zizzi’s for lunch, arriving at 2.20pm and leaving at 3.35pm.

Mr Skripal and his daughter were caught on CCTV leaving the restaurant on foot minutes later, and emergency services received the first report from members of the public who saw them slumped on a bench at 4.15pm.

Theresa May has named Russia as the likely culprit behind the attack, because of its development of Novichok and “record of conducting state-sponsored assassinations”, including against defectors like Mr Skripal who are viewed as legitimate targets.

“Either this was a direct act by the Russian state against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others,” she told MPs on Monday.

“This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals.

“It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk, and we will not tolerate such a brazen attempt to murder innocent civilians on our soil.”

Peter Wilson, the UK’s representative to the OPCW, told the watchdog’s executive council session that in “whichever scenario, Russia has failed, for many years to declare chemical weapons development programmes dating from the 1970s”.

“All of us in this room should be aware: if the norm against chemical weapons use continues to be eroded, if we don’t stand up to enforce the fundamental tenets of the Convention, what has happened in the United Kingdom could happen in any one of our countries,” he added.

“Those who have used chemical weapons cannot be immune from the consequences of their actions. We must all do all that we can to bring perpetrators of chemical weapons attacks to justice, whoever they are, and wherever they may be.”

Following a meeting of the Government's emergency Cobra committee on Tuesday, Amber Rudd confirmed that police and MI5 are reviewing allegations of Russian state involvement in up to 14 deaths on British soil.

“My priority is this incident, the investigation itself, so we get the information as quickly as possible, and also the safety and security of the people in the community,” the Home Secretary added.

The Russian government has denied involvent and demanded to see samples of the nerve agent itself (Getty)

"I know that international allies have begun to rally their support and make comments publicly but at the moment what we are doing is awaiting the Russian response before stepping up and responding as the Prime Minister has said we will."

Boris Johnson said it was “overwhelmingly likely” that the Russian government had orchestrated the first use of nerve agents in Europe since the Second World War.

“We’re giving Russia until midnight tonight to explain how it came to be that Novichok was used on the streets of Wiltshire,” he added.

“If they can come up with a convincing explanation then obviously we will want to see full disclosure of that to the Organisation for the Prohibition of Chemical Weapons in The Hague.

“If not, then clearly we will want to be announcing the UK response.”

The Russian government has denied involvement in the attack and said it will ignore the Prime Minister's midnight ultimatum unless it is given samples of the nerve agent used.

Mr Skripal was jailed for “treason” by Russia in 2006, after working as a double agent for MI6 during his time as a colonel in the GRU military intelligence service.

He was released early in 2010, following a high-profile “spy swap,” with Russian sleeper agents uncovered in the US and given refuge in the UK and British citizenship.

Mr Skripal appeared to lead a quiet life in Salisbury but sources told The Independent he may have come to the attention of powerful enemies in Russia by “freelancing” for private intelligence firms run by former MI5, MI6 and GCHQ agents.

They insisted he was not an active MI6 asset and was not viewed as being under threat.

Ms Skripal, 33, lived with her parents in Salisbury before her mother died in 2012, when the death was recorded as endometrial cancer.