Dawn Sturgess, 44, one of two UK citizens poisoned by Novichok in Amesbury, has died.
She may have picked up an item missed in the original decontamination operation in March.
Her partner, Charlie Rowley, 45, remains in hospital in critical condition.
- Second Novichok poisoning likely from item missed in decontamination
- Poisoned Britons may have had chance encounter with Novichok
- Second poisoning in Salisbury confirmed
Police have launched a murder enquiry.
"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," said assistant commissioner Neil Basu, the head of UK Counter Terrorism policing.
"Detectives will continue with their painstaking and meticulous work to gather all the available evidence so that we can understand how two citizens came to be exposed with such a deadly substance that tragically cost Dawn her life."
Police say there is no evidence Ms Sturgess and Mr Rowley visited any of the areas decontaminated after former Russian spy Sergei Skripal and his daughter Yulia were poisoned in March.
It is not yet known whether the nerve agent that killed Ms Sturgess was from the same batch that poisoned the Skripals.
An attempted murder investigation into the poisoning of the Skripals is on-going.
Police are investigating whether the two cases may be linked.
Newshub.
