Experts are trying to determine whether the Novichok that poisoned a couple in Wiltshire was from the same batch used in the attempted murder of Sergei Skripal and his daughter Yulia.
Scotland Yard said the nerve agent was found in a small bottle in the Amesbury home of Charlie Rowley, 45, who, along with partner Dawn Sturgess, 44, was exposed to the deadly substance last month.
Ms Sturgess died, while Mr Rowley is seriously ill in hospital.
Scientists at the nearby Defence, Science and Technology Laboratory at Porton Down, confirmed the substance in the bottle is Novichok.
But further tests will be carried out to try to establish whether it is from the same batch that poisoned the Skripals in March.
Counter-terror detectives are trying to establish where the bottle came from and how it came to be in Mr Rowley’s home.
The UK has invited experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) to independently confirm the identity of the nerve agent.
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.”
Ms Sturgess, a mother of three, died in hospital on Sunday night having been exposed to Novichok.
A post mortem is scheduled to take place on Tuesday and an inquest into her death is set to open and adjourn in Salisbury on Thursday.
Mr Rowley, who regained consciousness this week, remains in a serious but stable condition in Salisbury District Hospital.
About 100 counter-terror detectives are working on the investigation along with officers from Wiltshire Police.
Investigators have spoken to Mr Rowley and will speak to him again in a bid to find out how he and his partner were poisoned.
Police said this was being done in close consultation with the hospital and the doctors.
Wiltshire Chief Constable Kier Pritchard welcomed the development, describing it as “significant and encouraging”.
“I hope that it will further reassure our communities in both Amesbury and Salisbury that the investigation, although complex, is meticulous,” he said.
Public Health England reiterated its advice to members of the public and urged residents not to touch, or pick up, unfamiliar objects.