A British woman has died after she was poisoned by the same nerve agent that struck a former Russian spy in March and triggered a crisis in relations between Western capitals and Moscow.
Dawn Sturgess, 44, died on Sunday after being exposed to Novichok on June 30 in western England, 11 kilometres from where Russian double agent Sergei Skripal and his daughter were attacked with the same poison four months ago.
Police are investigating Sturgess' death as a murder, while British Prime Minister Theresa May says she's appalled and shocked by the death.
Police are investigating how Sturgess and a 45-year-old man, named by media as Charlie Rowley, came across an item contaminated with Novichok, which was developed by the Soviet military during the Cold War.
The March attack on the Skripals prompted the biggest Western expulsion of Russian diplomats since the Cold War as allies sided with Britain's view that Moscow was either responsible or had lost control of the nerve agent.
Moscow hit back by expelling Western diplomats.
Britain's interior minister Sajid Javid on Sunday said the "desperately sad news only strengthens our resolve to find out exactly what has happened."
The head of UK Counter Terrorism policing, Assistant Commissioner Neil Basu, said Sturgess, a mother of three, died as the result of "an outrageous, reckless and barbaric act."
The 45-year-old man remained critically ill in hospital.
The poisoning in March of the Skripals with Novichok was the first known offensive use of such a chemical weapon on European soil since World War Two.
Russia, which is currently hosting the soccer World Cup, has denied any involvement in the Skripal case and suggested the British security services had carried out the attack to stoke anti-Moscow hysteria.
The two Britons became unwell on June 30 in Amesbury, a town in southwest England, 11km from Salisbury, where Skripal and his daughter Yulia were attacked.
It was initially thought they had overdosed on heroin or crack cocaine but tests at the Porton Down military research centre showed they had been exposed to Novichok.
Britain has notified the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW).
Further tests of samples from Sturgess and the man showed they were exposed to the nerve agent after touching a contaminated item with their hands, police said on Sunday.
Javid said earlier on Sunday that police had a working hypothesis that the two poisoning incidents were connected. He also said there were no plans at this stage for further sanctions against Russia.
Sturgess died at Salisbury District Hospital, the same facility that nursed the critically ill Skripals.
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. Her father was discharged on May 18.
The hospital's medical director, Christine Blanshard, told the BBC that hospital staff worked tirelessly to save Sturgess. "They did everything they could," she said.
Britain's Counter Terrorism Policing Network is leading the investigation into the nerve agent attacks.
There was no evidence that the two Britons had visited any of the sites that were decontaminated following the attempted killings of Sergei and Yulia Skripal, police said on Sunday.