Video report by ITV News Security Editor Rohit Kachroo
Counter terrorism detectives believe they may have found the source of the deadly nerve agent Novichok which left one person dead and another seriously ill.
Mother-of-three Dawn Sturgess, 44, and her partner Charlie Rowley, 45, were exposed to the substance last month. Ms Sturgess died, while Mr Rowley remains seriously ill in hospital.
On Friday, New Scotland Yard revealed that searches of Mr Rowley’s home in Amesbury had turned up a small bottle - and that lab tests carried out by the nearby Defence, Science and Technology Laboratory at Porton Down confirmed it contained Novichok.
Former chemical weapons inspector Jerry Smith:
Jerry Smith, a former chemical weapons inspector, told ITV News the find was "important".
"A piece of evidence that was actually used in the attempted assassination may mean there is evidence for the police investigation to follow up, such as DNA or fingerprints.
"And of course, having some more of the material ... from a forensics point of view, with regards to the chemical analysis, would also be important."
The UK has now invited experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) to independently confirm the identity of the nerve agent.
Further scientific tests will be carried out to try to establish whether it is from the same batch that contaminated Sergei and Yulia Skripal in March.
Inquiries are also underway to establish where the bottle came from, and how it came to be in Mr Rowley’s home.
A post mortem examination of Ms Sturgess is scheduled to take place on Tuesday, while an inquest into her death is expected to open and adjourn in Salisbury on Thursday.
This is clearly a significant and positive development
Mr Rowley, who regained consciousness earlier in the week, remains in a serious but stable condition in Salisbury District Hospital.
Assistant Commissioner Neil Basu, head of UK Counter Terrorism Policing, called the discovery of the bottle a "significant and positive" development.
“However, we cannot guarantee that there isn’t any more of the substance left, and cordons will remain in place for some considerable time," he added.
“This is to allow thorough searches to continue as a precautionary measure for public safety and to assist the investigation team.
“I also appreciate there is a lot of interest in this, however, we are not in a position to disclose any further details regarding the bottle at this stage.
“The safety of the public and our officers remains paramount and we are continuing to work closely with Wiltshire Police, scientists, health experts from Public Health England and other partners.”
About 100 detectives from the Counter Terrorism Policing Network continue to work on the investigation, alongside colleagues from Wiltshire Police.
Officers have already spoken to Mr Rowley, and say they will be speaking to him again as they try to establish how he and his partner came to be contaminated.
Police said this contact was being done in close consultation with the hospital and the doctors.
Wiltshire Chief Constable Kier Pritchard welcomed the development, describing it as “significant and encouraging”.
He also revealed that private security guards would be joining officers on some of the cordons in place around the area, to free up PCSOs and police constables to return to day to day duties.
“We are now almost two weeks on from the initial incident in Amesbury and I continue to be overwhelmed by the resilience shown by our communities," he said.
Meanwhile, Public Health England reiterated its advice to members of the public and urged residents not to touch, or pick up, unfamiliar objects.