Counter-terrorism officers recovered a small bottle on Wednesday from the Amesbury home of Charlie Rowley, who is in a serious condition in hospital.
The incident came months after the poisoning of former Russian agent Sergei Skripal and his daughter Yulia in Salisbury, also with Novichok, prompting fears that Sturgess and Rowley had been exposed to some of the left-over poison.More news: Europe 'better watch themselves' on immigration
Charles Rowley is now conscious in hospital and police have found a bottle of Novichok at his house.
"However, we can not guarantee that there isn't any more of the substance left", said Assistant Commissioner Neil Basu, Head of UK Counter Terrorism Policing.
Basu said cordons would remain in place in some locations to protect the public. They were found to have been poisoned with Novichok, a lethal nerve agent produced in the Soviet Union during the Cold War.More news: North Atlantic Treaty Organisation summit exposes Juncker health problems
"This is clearly a significant and positive development".
'Further scientific tests will be carried out to try and establish whether it is from the same batch that contaminated Sergei and Yulia Skripal in March - this remains a main line of enquiry for police. "However, we can not guarantee that there isn't any more of the substance left and cordons will remain in place for some considerable time", Metropolitan Police Assistant Commissioner Neil Basu said in a statement.
The British government previously blamed the attempted assassination of Sergei Skripal on Russian Federation, which Russia denies.More news: Watch NASA's iconic Cape Canaveral launch towers crumble
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.
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- British man poisoned with Novichok is no longer in critical condition - hospital