The woman left critically ill after being exposed to a Russian nerve agent has died, Scotland Yard said.
Dawn Sturgess, 44, came into contact with Novichok in the town of Amesbury, Wiltshire.
Prime Minister Theresa May has said she is appalled and shocked by Ms Sturgess death.
Scotland Yard has opened a murder inquiry after Ms Sturgess died in hospital on Sunday evening.
Charlie Rowley, 45, who was also exposed to the substance, remains in a critical condition in hospital, police said.
They fell ill on June 30 after the exposure to the same type of nerve agent used to poison former Russian spy Sergei Skripal and his daughter Yulia in Salisbury In March.
Police suspect they were exposed through a contaminated item left over from the first attack, which Britain blames on Russia.
Russia has denied the allegation.
In a statement, the prime minister said: I am appalled and shocked by the death of Dawn Sturgess, and my thoughts and condolences go to her family and loved ones.
Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder.
The Government is committed to providing full support to the local community as it deals with this tragedy.
Assistant Commissioner Neil Basu, the head of UK Counter Terrorism 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.
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.
Dawns family has asked the media to kindly respect their privacy at this difficult time.