A 44-year-old British woman died on Sunday after being exposed to the Novichok nerve agent in western England just a few miles from where Russian ex-spy Sergei Skripal and his daughter were struck down by the same agent four months ago.
Police said they were continuing to investigate how Dawn 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.
The death of Sturgess was being investigated as a murder, police said in a statement.
Prime Minister Theresa May said she was appalled and shocked by the death.
The poisoning 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 March incident and suggested the British security services had carried out the attack to stoke anti-Moscow hysteria.
The two Britons were taken ill on June 30 in Amesbury, a town in southwest England, 11 km (7 miles) from Salisbury, where Skripal and his daughter Yulia were attacked in March.
Police said the 45-year-old man remained critically ill in hospital.
Assistant Commissioner Neil Basu, the head of UK Counter Terrorism policing, said Sturgess’ death was “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,” he said.
The two Britons were initially thought to have taken an overdose of heroin or crack cocaine.
But tests by 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).
Britain’s interior minister Sajid Javid said earlier on Sunday that police were still working to discover how the two individuals were exposed to the nerve agent.
They had a working hypothesis that the two poisoning incidents were connected, he said, although he added that there were no current plans for further sanctions against Russia.
The investigation is being led by detectives from Britain’s Counter Terrorism Policing Network and around 100 detectives are working round the clock alongside colleagues from Wiltshire police.