Police in the UK say a couple must have been exposed to a 'high dose' of the novichok nerve agent.
44-year-old Dawn Sturgess and her 45-year-old partner Charlie Rowley were taken to hospital over the weekend after exposure to the substance, which earlier this year was used against former Russian spy Sergei Skripal and his daughter Yulia in Salisbury.
Ms Strugess died in hospital yesterday evening, while Mr Rowley remains in a critical condition.
Police believe the couple may have been exposed to the nerve agent through handling a container.
A murder investigation has also been launched, with police saying they are not yet able to say whether or not the incident is linked to the attack on the Skripals.
Tests are being carried out at sites both Salisbury and nearby Amesbury, with the couple having spent time in both towns last Friday before taking ill on Saturday.
A van that Charlie travelled in is also being tested.
"Shocking and utterly appalling"
Picture by: Yui Mok/PA Wire/PA Images
Speaking today, Met Police Assistant Commissioner Specialist Operations Neil Basu explained: "It is both shocking and utterly appalling that a British citizen has died having being exposed to a Novichok nerve agent.
"We’re determined to find out how Dawn and her partner, Charlie Rowley, came into contact with such a deadly substance; and we will do everything we possibly can to bring those responsible to justice."
He added: "Their reaction was so severe, it resulted in Dawn’s death and Charlie being critically ill. This means that they must have got a high dose and our hypothesis is that they must have handled a container we are now seeking."
Nobody else has shown signs of being exposed to the nerve agent, including three other men who travelled in the van with Mr Rowley.
21 other people have come forward with concerns about potential exposure to the nerve agent, but police said they have all been screened and given the all-clear.
The British government has repeatedly blamed Russia for the incident in Salisbury - claims Moscow has vehemently denied.
The Skripals both survived being exposed to the nerve agent, and they have both been discharged from hospital.
