The nerve agent that killed a woman in Wiltshire and left her boyfriend critically ill has been found in his house, police say.
Officers recovered a small bottle on Wednesday from the Amesbury home of Charlie Rowley, who is in a serious condition in hospital.
Tests at Porton Down have confirmed that the substance in the bottle is novichok, Scotland Yard said.
An investigation is now under way to establish whether it is from the same batch that poisoned Sergei and Yulia Skripal in Salisbury in March.
Mr Rowley's partner Dawn Sturgess, 44, died on Sunday after falling ill on 30 June following exposure to the nerve agent.
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 UK has invited experts from the Organisation for the Prohibition of Chemical Weapons to independently confirm the identity of the nerve agent.
Detectives say they are trying to work out where the bottle came from and how it came to be in Mr Rowley's house.
Police have spoken briefly to the 45-year-old who is now conscious in hospital.
The death of Ms Sturgess, a mother-of-three, is being treated by police as murder.
A post-mortem will take place on Tuesday before an inquest into her death opens on Thursday.
About 100 counter-terrorism detectives are working on the investigation into the poisoning, along with officers from Wiltshire Police.
Authorities insist the risk to the public in Salisbury and Amesbury remains low and there have been been no further cases of people falling ill.
Former Russian spy Sergei Skripal and his daughter Yulia were found unconscious on a bench in Salisbury on 4 March after being exposed to novichok. They have since been discharged from hospital.
More from Amesbury poisoning
The government has blamed Russia for the poisonings, accusing Moscow of using Britain as a "dumping ground for poison".
Public Health England has reiterated its advice to members of the public and urged residents not to touch or pick up foreign objects.