Counter terrorism detectives investigating the contamination of two people by Novichok believe they have found the source of the deadly substance.
Dawn Sturgess, 44, died and her partner Charlie Rowley, 45, is seriously ill in hospital after they were exposed to the nerve agent last month.
New Scotland Yard said that on Wednesday a small bottle was recovered during searches of Mr Rowley’s home in Amesbury.
It was taken to the nearby Defence, Science and Technology Laboratory at Porton Down, Wiltshire for tests.
Following those tests, scientists have now confirmed the substance in the bottle is Novichok.
The UK has 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.
This remains a main line of inquiry for police.
Inquiries are under way to establish where the bottle came from and how it came to be in Mr Rowley’s home.
Ms Sturgess, a mother of three, died in hospital on Sunday night having been exposed to Novichok.
A post mortem is scheduled to take place on Tuesday and an inquest into her death is set to open and adjourn in Salisbury on Thursday.
Mr Rowley, who regained consciousness this week, remains in a serious but stable condition in Salisbury District Hospital.
Assistant Commissioner Neil Basu, head of UK Counter Terrorism Policing, said: “This is clearly 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.
“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 this investigation, alongside colleagues from Wiltshire Police.
Officers from the investigation team have spoken to Mr Rowley and will be speaking to him further 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”.
“I hope that it will further reassure our communities in both Amesbury and Salisbury that the investigation, although complex, is meticulous,” he said.
“We continue to support colleagues from the Counter Terrorism Policing Network to progress the inquiry as swiftly and safely as possible.
“The way that we do this might start to look slightly different from next week, when private security guards will join my officers on some of the cordons.
“This will free up some Wiltshire Police officers to get back to supporting day-to-day community policing.
“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.
“Despite this latest development from the investigation team, the guidance from Public Health England around picking up foreign objects hasn’t changed.
“It’s a highly precautionary measure and the overall risk to the public is low but their advice is simple - ‘If you didn’t drop it, don’t pick it up’.”
Public Health England reiterated its advice to members of the public and urged residents not to touch, or pick up, unfamiliar objects.