A mother-of-three has died after she and her partner were exposed to the nerve agent Novichok.
Scotland Yard said they have launched a murder investigation after 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 in Amesbury, Wiltshire, last Saturday.
The murder investigation is the second major probe involving the nerve agent this year, following the case of Sergei and Yulia Skripal who were left critically ill in March. Both have recovered.
Tests have revealed that Ms Sturgess and Mr Rowley were exposed to the nerve agent after touching a contaminated item.
The possibility that the two investigations might be linked is "clearly a key line of inquiry for police", the Met said.
Assistant Commissioner Neil Basu described Ms Sturgess' death as "shocking and tragic news" and said the force's thoughts were with the families of both victims.
"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," he added.
"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."
Novichok container not found
“This is the most deadly chemical weapon ever produced, the Russians have been developing it for over 30 years,” said Hamish de Bretton-Gordon, a chemical weapons expert.
“I understand [Ms Sturgess] had a dose as large as Skripal, and it is designed to kill you.
“We’re now fairly certain that they handled some sort of container, perhaps a syringe, which was probably how the novichok was transported around the UK.
“Once we find that, we can breathe a sigh of relief. It seems most likely that it’s in Queen Elizabeth Gardens in Salisbury.”
Prime Minister sent condolences
Ms Sturgess' family are being given support by specialist officers and have asked for privacy.
Prime Minister Theresa May said she was "appalled and shocked", and sent her "thoughts and condolences" to the family.
"Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder," she added.
"The Government is committed to providing full support to the local community as it deals with this tragedy."
A post-mortem examination will be scheduled to take place in due course.
Public risk ‘remains low’
Wiltshire Police Chief Constable Kier Pritchard said: "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."
Public Health England (PHE) said their assessment "remains that the overall risk to the general public remains low".
PHE director Paul Cosford said: "As a precaution we still advise the public not to pick up any strange items such as needles, syringes or unusual containers."
Home Secretary Sajid Javid accused Russia of using Britain as a "dumping ground for poison".
The Russian Embassy hit back, accusing the Government of trying to "muddy the waters" and "frighten its own citizens".