A British woman died on Sunday after being exposed to novichok nerve agent in western England, a few miles from where Russian former double agent Sergei Skripal and his daughter were struck by the same compound four months ago.
Police said they were investigating how Dawn Sturgess, 44, and a 45-year-old man – named by media as Charlie Rowley – came across an item contaminated with novichok, a chemical weapon 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 claims 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.
Prime Minister Theresa May said she was appalled and shocked by the death.
Home Secretary Sajid Javid said: “The death of Dawn Sturgess is shocking and tragic news, and I want to express my sincere condolences to her family and friends. This has now become a murder investigation, and police and security officials are working around the clock to establish the full facts.
“This desperately sad news only strengthens our resolve to find out exactly what has happened.
“As I said earlier today when I visited Amesbury and Salisbury, the government will continue to provide the local community all the support it needs.”
The Conservative MP for Salisbury and South Wiltshire, John Glen, said that he was “deeply saddened to hear that Dawn Sturgess has died at Salisbury District Hospital. I can assure the people of South Wiltshire that the police will be given all necessary resources to find out exactly what has transpired and bring those responsible to justice.”
Russia, which is hosting the World Cup, 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, 11 kilometres from Salisbury, where Mr Skripal and his daughter Yulia were taken ill in March.
Police said Mr Rowley remained critically ill in hospital.
London Metropolitan Police Assistant Commissioner Neil Basu, the head of UK counter terrorism policing, said Ms Sturgess’s 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.
Tests by Porton Down chemical weapons 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).
Mr Javid said 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. 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 about 100 officers are working around the clock alongside Wiltshire police.