A women has died after being exposed to the nerve agent Novichok in Amesbury, Wiltshire, Scotland Yard said.
Prime Minister Theresa May said she was “appalled and shocked” by the death of 44-year-old Dawn Sturgess.
Officers have launched a murder investigation.
In a statement, 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.
“Dawn’s family has asked the media to kindly respect their privacy at this difficult time.”
Prime Minister 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.”