Police have located the source of the Novichok nerve agent responsible for poisoning two people in Salisbury last month.
Counter terrorism investigators recovered a small glass bottle from the home of victim, Charlie Rowley, which has tested positive for the deadly chemical.
Further tests on the bottle and its contents are now being carried out, but it is hoped it could also provide crucial evidence to prove who attacked former Russian double agent, Sergei Skripal and his daughter, Yulia, in March.
Dawn Sturgess, 44, from Durrington, Wiltshire died in hospital on Sunday evening after being exposed to the nerve agent the previous weekend.
Her partner, Charlie Rowley, 45, was also taken to hospital critically ill, but has since regained consciousness and has been interviewed by police.
It is thought Mr Rowley was able to help detectives when he emerged from his coma as to the whereabouts of the source of the contamination.
Salisbury three way split
Detailed examination of the bottle and its contents will now take place as part of the murder investigation that was launched following the death of Ms Sturgess.
Police have also expressed their determination to track down those who carried out the assassination attempt on Mr Skripal, although the blame has been laid firmly at the door of the Russian state.
A Scotland Yard spokesman 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 (DSTL) 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.
“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.”
Wiltshire Chief Constable, Kier Pritchard, described the development as "significant and encouraging", but urged locals to remain vigilant and not to pick up any litter that they had not dropped themselves."
He said: “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’. This is particularly important as we move into the school holidays."
Meanwhile a father of two from north Wales was questioned over the Novichok poisonings after detectives found suspicious chemicals in his shed.
David James, 36, was held in custody for nine hours after officers discovered dozens of test tubes and laboratory equipment during a search of his home.
But it turned out the chemicals were harmless ingredients he was using to make soap in order to get his daughters interested in science.