A 44-year-old woman, Dawn Sturgess, who was exposed to nerve-agent, Novichok, in southwest England has died, police said.
She was one of the two people who fell ill last weekend in Amesbury, near Salisbury where a Russian spy was targeted by the same type of chemical in March.
PM Theresa May said she was "appalled and shocked" by the death of Sturgess.
"Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder," May said.
She also offered her condolences to Sturgess's relatives.
Sturgess, along with a man, was believed to have been exposed to Novichok by handling a container; a link to the Salisbury attack in March is a main line of investigation.
The Salisbury incident left former Russian spy Sergei Skripal and his daughter Yulia, who was visiting from Russia, in a critical condition; they later recovered. Britain and its allies blamed Moscow for the incident, prompting angry denials that led to an international diplomatic crisis.
Assistant Commissioner Neil Basu, Britain's counter-terror police head, said, "The 45-year-old man Charlie Rowley who fell ill with Dawn remains critically ill in hospital and our thoughts are with him."
"This terrible news has only served to strengthen our resolve to identify and bring to justice the persons responsible for what I can only describe as an outrageous, reckless and barbaric act," he added.
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