A 44-year-old British woman has died after being exposed to the Novichok nerve agent in western England just a few miles from where Russian ex-spy Sergei Skripal and his daughter were struck down four months ago.
Police had been working to discover how Dawn Sturgess and a 45-year-old man had come across an item contaminated with Novichok, which was developed by the Soviet military during n the Cold War.
The two Britons had been taken ill last Saturday.
Prime Minister Theresa May said she was appalled and shocked by the death of Sturgess.
Britain has accused Russia of poisoning the Skripals with Novichok in what is the first known offensive use of such a chemical weapon on European soil since World War Two.
Russia, currently hosting the soccer World Cup, has denied any involvement in the March incident and suggested the British security services had carried out the attack to stoke anti-Moscow hysteria.
Reporting by Paul Sandle, writing by Guy Faulconbridge; Editing by Keith Weir