A woman who was exposed to the nerve agent novichok in Amesbury has died in hospital.
Police have launched a murder inquiry after Dawn Sturgess, 44, died on Sunday evening at Salisbury District Hospital, Scotland Yard said.
The mother of three was admitted to hospital after falling ill at her partner's home in Amesbury, near Salisbury, last Saturday.
Her partner, Charlie Rowley, remains in hospital in a critical condition after the pair were exposed to novichok.
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."
Police said nobody else has come to hospital with the same symptoms as the couple.
Dr Christine Blanshard, medical director at Salisbury District Hospital, expressed her condolences to Ms Sturgess' family and said her staff "did everything they could".
"The staff here at Salisbury District Hospital worked tirelessly to save Dawn," she said.
"Our staff are talented, dedicated and professional and I know today they will be hurting too.
"They did everything they could."
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."
Wiltshire Police constable Kier Pritchard said "today is the day we hoped would never come" as he urged those with questions to speak to officers patrolling the community.
Professor Paul Cosford, from Public Health England, said the public is still being advised not to pick up "any strange items such as needles, syringes or unusual containers".
"The overall risk to the general public remains low," he added.
If anybody was in the locations identified by police from 10pm on Friday 29 June they should wash their clothes in a washing machine and "keep your items double-bagged and securely fastened, if they are dry clean only," he added.
Ms Sturgess collapsed at about 10.15am on 30 June and was taken to hospital in an ambulance.
Mr Rowley was then taken to hospital at 3.30pm the same day, which is when police were informed.
Scientists at the UK's chemical laboratory Porton Down confirmed on Wednesday that the couple had been exposed to novichok.
Russian double agent Sergei Skripal and his daughter Yulia Skripal were poisoned with the nerve agent earlier this year in Salisbury but managed to recover after spending months in Salisbury District Hospital.
Police suspect Ms Sturgess and Mr Rowley were exposed to novichok through a contaminated item left over from the first attack.
Britain has blamed Russia for the attack on the Skripals, but the Kremlin denies the allegation.
Detectives are still investigating the attempted murders of the Skripals.
They are looking into whether the separate incidents may be linked.
But officials said there is no evidence that the pair visited any of the sites in Salisbury which were decontaminated after the attempted murders of the Skripals.
Six locations visited by the couple in Salisbury and Amesbury before they fell ill have been cordoned off, including Mr Rowley's home on Muggleton Road in Amesbury, a Boots shop and a baptist church.
Deeply saddened to hear that Dawn Sturgess has died at Salisbury District Hospital. I can assure the people of South Wiltshire that the police will be given all necessary resources to find out exactly what has transpired and bring those responsible to justice.
— John Glen MP (@JohnGlenUK) July 8, 2018
Officers also evacuated the hostel in Salisbury where Ms Sturgess lived and investigators in hazmat suits were seen swabbing the building and collecting evidence on Friday.
CCTV footage has emerged of Ms Sturgess and Mr Rowley buying alcohol more than 24 hours before they became seriously unwell.
The couple, who were known drug users, spent Friday afternoon and evening in Salisbury and visited a number of shops.
Salisbury MP John Glen sought to assure his constituents that police were working hard to find out who was responsible.
Labour leader Jeremy Corbyn said he was "shocked" and that his thoughts were with the victim's friends and family.
More from UK
"A full and thorough police investigation must now establish the facts, provide support to the local community and bring those responsible to justice," Mr Corbyn added.
:: If anyone has any information which could help with the investigation call police on 0800 789 321.