Police have spoken to a man exposed to novichok in Wiltshire for the first time after he regained consciousness.
Investigators hope that Charlie Rowley, whose partner died in hospital on Sunday, can help trace the container of nerve agent that poisoned them.
A spokesperson for the Metropolitan Police said: “Officers from the investigation team have spoken briefly to Charlie and will be looking to further speak with him in the coming days as they continue to try and establish how he and Dawn came to be contaminated with the nerve agent.
“Any contact officers have with Charlie will be done in close consultation with the hospital and his doctors.”
Mr Rowley regained consciousness at Salisbury District Hospital on Tuesday, two days after Dawn Sturgess died.
The director of nursing, Lorna Wilkinson, said the 45-year-old was no longer in a critical condition after progress overnight and described him as “serious but stable”.
“Our staff will continue to work hard to provide the care that Charlie needs,” she added. “He still has some way to go to recover, but the progress we’ve seen so far gives us cause for optimism.”
Police have warned that novichok can remain deadly for 50 years in a sealed container, amid concern that other members of the public could be accidentally exposed to the chemical weapon.
Assistant Commissioner Neil Basu, the head of Counter terrorism policing in the UK, admitted he could not guarantee that there are no further traces left.
“I would love to be able to stand here and say how we have identified and caught those responsible and how we are absolutely certain there are no traces of nerve agent left anywhere in the county,” he told a public meeting in Amesbury.
“The brutal reality, however, is that I cannot offer you any such assurances or guarantees at this time.”
Police believe Ms Sturgess and Mr Rowley unwittingly picked up a container of novichok and have cordoned off their homes and other locations they visited in Salisbury and Amesbury on 29 and 30 June.
The possibility that the substance is the same batch of nerve agent used against Sergei Skripal and his daughter in March is their main line of inquiry, but a forensic link has not yet been found and scientists at the Porton Down defence laboratory are still running tests.
“This is a very rare substance banned by the international community and for there to be two separate distinct incidents in one, small English county is implausible to say the least,” Mr Basu said.
“We believe that Dawn and Charlie handled some kind of container which the nerve agent was in, and we’re focusing our efforts on finding this container…if it was sealed in a container and it was in a landfill site it would effectively be safe because it would not be touched by anyone and it would last – I’ve been told by scientists – 50 years.”
Mr Basu said the container has not been found, adding that he has “no idea what it looked like”.
When a member of the public asked whether police were “looking for a needle in a haystack”, he replied: “Yes, that’s why we need witnesses or intelligence that tells us where this may be…the brutal fact is we don’t know where they found it.
“I am hoping Charlie recovers, and when he recovers he will be able to tell us and perhaps shed some light on it, which will narrow our search dramatically.”
Health authorities have told residents in the Salisbury and Amesbury areas not to pick up “any foreign object which could contain liquid or gel” as searches continue.
Dame Sally Davies, the chief medical officer, said: “Do not pick up containers, syringes, needles, cosmetics or similar objects, made of materials such as metal, plastic or glass.
“This is particularly important as families are starting to prepare for their children’s summer holidays and so I am asking that people are extra vigilant.”
A member of the audience at Tuesday night’s meeting suggested that authorities had not been actively looking for the source of novichok used against the Skripals until the latest incident.
“I take your point and I know you are really concerned about it,” Mr Basu replied.
“This is the reason the most important line of inquiry for the Skripals and the same for Dawn and Charlie is exactly where they have been at any one time.
“The second-most important line of inquiry is finding the people responsible.”
Officials said the couple had not visited areas of Salisbury that had been frequented by the Skripals and subsequently decontaminated, and insisted that the overall threat to the wider public remains low.
More than 20 people, including a police officer, have presented themselves to authorities with concerns but all have tested negative for nerve agent exposure.
Baroness Jane Scott, leader of Wiltshire County Council, said people were “reassured” by the meeting but they remained concerned over the whereabouts of the missing novichok container.
She told BBC Breakfast: “There was a great resilience in the meeting last night and when people went away they didn’t seem to be scared.
“They were concerned, they want answers, but they realise that those answers will come when the professionals have done their job.”
A car belonging to an emergency services worker has been seized for testing in Swindon, as well as a red Ford Transit van Mr Rowley travelled in before falling ill.
A bus that Ms Sturgess and Mr Rowley used to travel from Salisbury to his home in Amesbury the night before they collapsed has also tested negative for novichok.
Relatives of Ms Sturgess, who had three children, have urged the public to “stop speculating” about her death and paid tribute to a “gentle soul who was generous to a fault”.
Emergency services initially treated the couple’s illness as drug-related – a response that was defended as “entirely proportionate” last week by Wiltshire Police Chief Constable Kier Pritchard.
But medics at Salisbury District Hospital became concerned about Ms Sturgess and Mr Rowley’s symptoms and sent samples to the Porton Down defence laboratory for analysis.
Defence secretary Gavin Williamson blamed Russia for committing “an attack on British soil” over the latest poisonings.
“The simple reality is that Russia has committed an attack on British soil which has seen the death of a British citizen,” he told MPs.
The Kremlin has denied any involvement in the attack on former double agent Mr Skripal or the latest incident, claiming that linking Russia to the poisoning would be “absurd”.
The government is awarding more than £5m of extra funding for recovery efforts in Salisbury and Amesbury, bringing the total package to more than £7.5m.
The funds include money to cover Wiltshire Police’s costs, revive tourism and supplement income lost by local businesses affected by shifting police cordons and public health fears.
Theresa May said: “The people of Salisbury and Amesbury have shown incredible resilience, both since March and in light of last week’s tragic events.
“The government will stand in full support of the local community and this funding package will ensure ongoing response and recovery needs are met.
“We will continue to work closely with local partners in south Wiltshire on the long-term recovery from these terrible incidents.”