Police have found a bottle believed to have contained the novichok that killed Dawn Sturgess and poisoned Charle Rowley in Wiltshire, Scotland Yard have announced.
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.
Her partner, 45-year-old Charlie Rowley, was also taken to hospital critically ill but has since regained consciousness.
He declined to give further details on the bottle, but police said inquiries were ongoing to establish where the bottle came from and how it came into Rowley's possession.
In a statement, police said: "On Wednesday 11 July, a small bottle was recovered during searches of Charlie Rowley's house in Amesbury".
Police said they had been able to speak to him "briefly".
Basu said cordons would remain in place in some locations to protect the public despite the apparent breakthrough in the case. "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", he added.
The Foreign Office also on Friday said it had invited independent technical experts from the global chemical weapons watchdog to Britain early next week "to independently confirm the identity of the nerve agent". The organization has the power to assign blame for chemical weapons use.
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. They were found to have been poisoned with Novichok, a lethal nerve agent produced in the Soviet Union during the Cold War.
The Skripals survived and have been released from a hospital, but are in a secret protected location.
"The risk to the public in Salisbury and Amesbury remains low".