A mother-of-three has died after she and her partner were exposed to the nerve agent novichok in Amesbury.
Dawn Sturgess, 44, died In hospital on Sunday evening.
Her partner Charlie Rowley, 45, is still in a critical condition after the couple fell ill on June 30.
Scotland Yard have launched a murder inquiry - the second major investigation involving the nerve agent this year, following the case of Sergei and Yulia Skripal in March.
Prime Minister Theresa May said she was "appalled and shocked" by the death.
Tests have revealed that the couple were exposed to novichok after touching a contaminated item with their hands.
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.
Dr Christine Blanshard, medical director at Salisbury District Hospital where Ms Sturgess died, said it was "desperately sad" and she expressed condolences to the 44-year-old's family.
She said: "The staff here at Salisbury District Hospital worked tirelessly to save Dawn. Our staff are talented, dedicated and professional and I know today they will be hurting too.
"They did everything they could."
Mr Rowley is also being treated at the hospital.
Prime Minister Theresa May 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."
CCTV shows Dawn Sturgess shopping in Salisbury the day before falling ill
In a statement, 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.
"Dawn's family has asked the media to kindly respect their privacy at this difficult time."
In a statement, Chief Constable Kier Pritchard, for Wiltshire Police said: “Today is the day we hoped would never come.
"It is with great sadness that we have learned of the tragic death of 44 year old Dawn Sturgess, as reported by AC Neil Basu and the Prime Minister.
"On behalf of all officers, staff and volunteers working for Wiltshire Police I wish to pass on our condolences to the family and friends of Dawn at this terrible time.
“I cannot begin to imagine the pain and suffering they must be feeling, coupled with all the questions they need answers to.
“I know this news will affect more people than just those who knew Dawn – it will affect our communities right across the county.
"Understandably, this is likely to raise the level of concern in Amesbury and Salisbury.
“I would like to remind our communities that this tragic development has not changed the Public Health England advice, which is that the risk to the wider public remains low. "