British police said Friday they have found a "small bottle" containing the Soviet-made nerve agent Novichok in the home of one of two Britons who fell ill from the substance last month.
"On Wednesday, 11 July, a small bottle was recovered during searches of Charlie Rowley's house in Amesbury," police said in a statement, four months after a former Russian spy and his daughter were poisoned with the same substance.
"Scientists have now confirmed to us that the substance contained within the bottle is Novichok," the statement said, adding that the test were carried out at the Porton Down defence laboratory which first identified the substance.
Rowley, 45, and his partner Dawn Sturgess, 44, collapsed at his house within hours of each other on June 30.
Sturgess died on Sunday, while Rowley has regained consciousness and is no longer in a critical condition.
Police said they had been able to speak to him "briefly".
The incident came after former spy Sergei Skripal and his daughter Yulia, who have both since recovered, collapsed on a bench on March 4 in the nearby city of Salisbury, southwest England, sparking an international diplomatic crisis after Britain blamed Russia for the attack.
Moscow has strongly denied any involvement in the poisoning.
The police said further testing was trying to establish whether the substance that affected Rowley and Sturgess was from the same batch as was used against the Skripals.
"Inquiries are under way to establish where the bottle came from and how it came to be in Charlie's house," police said.
Neil Basu, head of UK counter-terrorism police, which are heading up the probe, said the discovery was "a significant and positive development".
"However, we cannot guarantee that there isn't more of the substance left and cordons will remain in place for some considerable time," he said.
"This is to allow thorough searches to continue as a precautionary measure for public safety and to assist the investigation team."
The Foreign Office also on Friday said it had invited independent technical experts from the international chemical weapons watchdog to Britain early next week "to independently confirm the identity of the nerve agent".
"During their visit they will be able to collect samples to inform this work. These samples will be analysed at highly reputable international laboratories designated by the OPCW (Organisation for the Prohibition of Chemical Weapons)", the foreign ministry said in a statement.
The police said further testing was trying to establish whether the substance that affected Rowley and Sturgess was from the same batch as was used against the Skripals