Dawn Sturgess, a mother of three, died on Sunday evening the Met said in a statement shortly before 10pm.
"This bad 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", Neil Basu, the head of United Kingdom counterterrorism policing, said in a statement.
Fire trucks and emergency response vechiles are parked outside a residential house in Amesbury, southern England, July 7, 2018, where a couple was exposed to the same nerve agent as the Russian double agent Sergei Skripal.
"We are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to", police said. Britain blames Russian Federation for the attack, however, Russian Federation has denied the allegation.
The man who became sick is still critically ill and in the hospital.
"Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder", she said.
Ms Sturgess's family are being given support by specialist officers and have asked for privacy.
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"The Government is committed to providing full support to the local community as it deals with this tragedy", she wrote in a second tweet.
Meanwhile the investigation into the attempted murders of the Skripals is ongoing as detectives continue to assess all the evidence available.
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.
They spent weeks in critical condition, but have both been discharged from Salisbury District Hospital, the same hospital where Sturgess died.
The British couple were believed to have become exposed to Novichok by handling a "contaminated item", with speculation that it could have been the container used to administer the nerve agent to the Skripals.
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Sergei Skripal and his daughter Yulia were poisoned by a nerve agent in March. The hospital's medical director, Dr. Christine Blanshard, said the staff "worked tirelessly to save Dawn".
Britain's public health authority acknowledged on Friday the concerns of people living in the area after the two incidents involving Novichok, but said it was confident that the risk to the public remained low.