A British woman has died after being exposed to the nerve agent Novichok in western England, the Metropolitan Police said in a statement on Sunday.
Dawn Sturgess, 44, and her partner are thought to have handled a "contaminated item" in Salisbury on 29 June, before falling ill in Amesbury the following morning.
Police said her relatives have been informed and are receiving support from specially trained family liaison officers.
Ms Sturgess' partner, 45-year-old Charlie Rowley, remains critically ill at Salisbury District Hospital.
Assistant Commissioner Neil Basu, the head of UK counterterror policing said: “This is shocking and tragic news. Dawn leaves behind her family, including three children, and our thoughts and prayers are with them at this extremely difficult time.
“The 45-year-old man who fell ill with Dawn remains critically ill in hospital and our thoughts are with him and his family as well.
“This terrible news has only served to strengthen our resolve to identify and bring to justice the person or persons responsible for what I can only describe as an outrageous, reckless and barbaric act.