British Police say they have found a container they believe was the source of the nerve agent Novichok that killed one woman and left her partner in a serious condition in hospital.
Dawn Sturgess, 44, died after coming in contact with the toxin. Her partner Charlie Rowely has regained consciousness but remains in a serious condition in Salisbury hospital.
"A small bottle was recovered during searches of Charlie Rowley's house in Amesbury," counter-terrorism police said in a statement.
"Scientists have now confirmed to us that the substance contained within the bottle is Novichok."
More tests were being carried out to try to establish whether it was from the same batch that contaminated former Russian spy Sergei Skripal and his daughter Yulia and inquiries were under way to establish where the bottle came from, the statement added.
Police warn they can't be certain there are no more contaminated sites.
"This is clearly a significant and positive development. However, we cannot guarantee that there isn't any more of the substance left," said Assistant Commissioner Neil Basu, head of UK counter-terrorism policing.
Police say they will question Mr Rowely to try to determine why he had the bottle in his home.
Britain and its allies blamed Russia for the attack in March on the Skripals, prompting the biggest Western expulsion of Russian diplomats since the Cold War. Moscow has rejected the accusations and has hit back by expelling Western diplomats.
The British Foreign Office said Britain had invited independent technical experts from the international chemical weapons watchdog to travel to the UK early next week to independently confirm the identity of the nerve agent.
An inquest into Ms Sturgess's death is set to open in Salisbury next Thursday.