Prime Minister Theresa May said she was "appalled and shocked" by the death of Dawn Sturgess, one of two people who fell ill last weekend in Amesbury, near the town of Salisbury on Sunday.
"Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder," she said.
She offered her condolences to the relatives and loved ones of Sturgess, 44, who had three children. She became ill alongside a man named locally as Charlie Rowley, 45.
They were believed to have become exposed to Novichok by handling a container, and a link to the Salisbury attack in March is a main line of investigation.
That incident left former Russian spy Sergei Skripal and his daughter Yulia, who was visiting from Russia, in a critical condition although they later recovered.
Britain and its allies blamed Moscow for trying to kill the pair, prompting angry denials that led to an international diplomatic crisis.
Assistant Commissioner Neil Basu, the head of Britain's counter-terror police, said Sturgess' death was "shocking and tragic news".
"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 in a statement.
"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."