Novichok that poisoned a couple in Wiltshire came from a small bottle found in the home of one of the victims, police say.
A bottle was found in a search at Charlie Rowley’s Amesbury house and was tested by scientists at Porton Down, the Metropolitan Police said.
Mr Rowley, 45, remains in hospital in Salisbury in a serious but stable condition after falling ill on 30 June.
His partner Dawn Sturgess, 44, died last weekend.
Scientists at the Defence, Science and Technology Laboratory are still trying to establish whether the deadly substance found at Mr Rowley’s house came from the same batch of Novichok that contaminated Sergei and Yulia Skripal in March.
Police said they were still trying to find out where the bottle came from, and why it ended up in the house.
Assistant Commissioner Neil Basu, the head of UK Counter Terrorism Policing, said it was “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,” he added.
“This is to allow thorough searches to continue as a precautionary measure for public safety and to assist the investigation team.”