London's Metropolitan Police force said detectives had become a murder investigation with 44-year-old Dawn Sturgess's death at a hospital in Salisbury.
A man named as Charlie Rowley, 45, also fell ill at the weekend and he remains in a critical condition.
Salisbury District Hospital treated Sturgess and Rowley.
Police said the investigation "into the attempted murders of the Skripals is ongoing as detectives continue to assess all the evidence available".
"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.
More than 100 police officers are working to try and locate a small vial believed to have contained the nerve agent Novichok, which was manufactured in the Soviet Union during the Cold War.
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Britain has openly accused Russian Federation of being responsible for poisoning the Skripals with Novichok, a family of advanced nerve agents developed by the Soviet Union in the 1970 and 1980s.
The man who became sick is still critically ill and in the hospital.
Dozens of detectives are working on the investigation, which is being led by the Counter Terrorism Policing Network, as well as officers from the Wiltshire Police Department.
"We are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to".
Confirmation from authorities Wednesday that two British citizens were exposed to Novichok shook residents around Salisbury, who thought a months-long cleanup had removed any threat from the powerful nerve agent.
Yulia Skripal, Sergei's daughter, was in a coma for 20 days after she was attacked and was eventually discharged about five weeks after the poisoning.
Public Health England said the risk to the general public "remains low".
Hospital officials said late Saturday that a number of people including a police officer had sought medical advice in the last week but had been found not to need any treatment.