British detectives have launched a murder inquiry after a woman who was exposed to the nerve agent Novichok in Amesbury, Wiltshire, died in hospital on Sunday evening, London police announced.
She has been formally identified as 44-year-old Dawn Sturgess, from Durrington. Her family has been informed and is receiving support from specially trained family liaison officers.
A post-mortem will be scheduled to take place in due course. British Prime Minister Theresa May said she was "appalled and shocked" by the death.
The Scotland Yard said a 45-year-old man who was also taken ill following exposure to the nerve agent remains critically ill in hospital.
It said the investigation is led by detectives from the Counter Terrorism Policing Network and around 100 detectives are working round the clock alongside colleagues from Wiltshire police.
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," he said.
Basu said detectives will continue with their painstaking and meticulous work to gather all the available evidence.
According to the police, at approximately 10:15 am on Saturday, 30 June, the South West Ambulance Service was called to a residential address in Amesbury, where Sturgess collapsed. She was subsequently taken hospital.
At around 3:30 pm on the same day, the ambulance service was called back to the same address after the 45-year-old man had fallen ill. He was also taken to hospital and Wiltshire police informed.
The Defense Science and Technology Laboratory at Porton Down confirmed to police on Wednesday, 4 July that Sturgess and her friend had been exposed to Novichok.
Further tests of samples from Dawn and the man showed that they were exposed to the nerve agent after touching a contaminated item with their hands.
The Metropolitan Police said Detectives are working as quickly and as diligently as possible to identify the source of the contamination, but this has not been established at this time.
Scotland Yard said there is no evidence that they visited any of the sites that were decontaminated following the attempted murders of Sergei and Yulia Skripal in March this year.
"We are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to," it said.
The investigation into the attempted murders of the Skripals is ongoing as detectives continue to assess all the evidence available.