Detectives investigating the poisoning of two people in the UK believe the source of the nerve agent was a small bottle found in one of the victim's house in Amesbury, Scotland Yard has said.
Two people were exposed to the nerve agent Novichok in Amesbury on 30 June.
Dawn Sturgess, 44, died in hospital at the weekend and her partner Charlie Rowley remains critically ill in hospital.
In a statement, Scotland Yard said: "On Wednesday 11 July, a small bottle was recovered during searches of Charlie Rowley's house in Amesbury. It was taken to the Defence, Science and Technology Laboratory at Porton Down, Wiltshire, for tests.
"Following those tests, scientists have now confirmed to us that the substance contained within the bottle is Novichok. Further scientific tests will be carried out to try and establish whether it is from the same batch that contaminated Sergei and Yulia Skripal in March - this remains a main line of enquiry for police.
"Inquiries are under way to establish where the bottle came from and how it came to be in Charlie's house."
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."
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Private security guards will be used by police on cordons from next week to "free up some Wiltshire Police officers to get back to supporting day-to-day community policing", Wiltshire Police Chief Constable Kier Pritchard 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've also been receiving support from other forces in the country with scene-guarding and I am incredibly grateful for this."