The mother poisoned by Novichok in Britain has died in hospital after suffering massive heart failure.
Dawn Sturgess, 44, was infected by the deadly nerve agent after picking up a contaminated item near where ex-spy Sergei Skripal was found unconscious in Salisbury.
Sturgess and her partner Charlie Rowley, 45, were admitted to hospital last week after being infected by the deadly substance.
Rowley remains critically ill in hospital.
A neighbour claimed the couple were exposed to the agent at Queen Elizabeth Gardens in Salisbury - just a four-minute walk away from the bench outside The Maltings shopping centre where the Skripals were found poisoned earlier this year.
The couple, both known to struggle with alcohol and drug addiction, were treated at Salisbury District Hospital.
A police officer was also treated for symptoms there, but was last night given the all clear by medics. Sturgess's son Ewan claims her heart stopped beating and she was starved of oxygen for half an hour as she fought for her life.
Prime Minister Theresa May said she was 'appalled and shocked' by the death of Dawn Sturgess.
The woman's family has been informed and is receiving support from the police.
A post-mortem will be scheduled to take place in due course.
The investigation is being led by detectives from the Counter Terrorism Policing Network and around 100 detectives are working round the clock alongside colleagues from Wiltshire police.
Head of UK Counter Terrorism policing Neil Basu 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.
'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.
He has previously hit out at her treatment, saying she is not being dealt with as a priority, like the Skripals, because she is a 'nobody alcoholic'.
Wiltshire Police is now dealing with the second major investigation involving the deadly nerve agent this year, after the poisoning of Sergei and Yulia Skripal in March.
Salisbury District Hospital also treated Mr Skripal, 67, his daughter Yulia, 33, as well as police officer Nick Bailey, who fell ill after he came to assist them.
All three eventually survived, with Mr Skripal the last to leave hospital, on May 18.