A woman who died after being exposed to the nerve agent Novichok in Wiltshire is believed to have had a “high dose” of the substance, police have said.
Mother-of-three Dawn Sturgess, 44, died on Sunday evening after falling ill in Amesbury on 30 June.
Her partner, Charlie Rowley, 45, who was also exposed to the nerve agent, remains critically ill in hospital.
Police are continuing to hunt for a contaminated container which they believe was handled by the pair.
A murder inquiry has been launched into Ms Sturgess’s death and a post-mortem examination is due to take place.
Home Secretary Sajid Javid chaired a meeting of the government’s emergency committee Cobra on Monday and is expected to address MPs later.
Officers have found a red Ford Transit van which Mr Rowley travelled in with three other men before falling ill. The vehicle has been sent to the government’s military research facility Porton Down.
Assistant Commissioner Neil Basu, the head UK counter-terrorism policing which is leading the investigation, said he was “unable to say” if the incident in Amesbury is linked to the poisoning of the Skripals on 3 March – but it is their “main line of inquiry”.
Russian former spy Sergei Skripal and his daughter Yulia fell ill after being poisoned with Novichok in Salisbury. Ms Skripal was discharged from Salisbury District Hospital in April, and her father in May.
A police officer, Det Sgt Nick Bailey, was also left seriously ill after responding to the attack on the Skripals and spent nearly three weeks in hospital.
Mr Basu said: “In the four months since the Skripals and Nick Bailey were poisoned, no other people besides Dawn and Charlie have presented with symptoms, but their reaction was so severe it resulted in Dawn’s death and Charlie being critically ill.
“This means they must have got a high dose and our hypothesis is that they must have handled a container that we are now seeking.”
PM ‘appalled and shocked’
Ms Sturgess, from Durrington, was rushed to hospital on Saturday 30 June after becoming ill at a house in Muggleton Road in Amesbury.
She has two sons, aged 19 and 23, and an 11-year-old daughter.
Theresa May said she was “appalled and shocked” by her death. Speaking to MPs in the House of Commons, she also paid tribute to the “dedication of the staff” at Salisbury District Hospital and thanked them for their “tireless work”.
Kremlin spokesman Dmitry Peskov said Russia was “deeply concerned” about the incident in Amesbury – but said accusations that it was involved are “absurd”.
The UK government previously blamed Russia for the poisoning of the Skripals in March and say the nerve agent used was a type developed by Russia. Russian authorities denied any involvement in the poisoning.
Mr Basu said detectives will continue with their “painstaking and meticulous work to gather all the available evidence”.
By Gordon Corera, BBC security correspondent
The death of Dawn Sturgess may not significantly change the mechanics of the police investigation on the ground but it will increase the pressure surrounding it.
A hundred detectives are already working round the clock to try to establish how Dawn Sturgess and Charlie Rowley were contaminated by Novichok.
Swabs taken from them in hospital revealed the highest level of contamination on their hands, leading to the theory they picked something up.
It may well be that it was Dawn who came into first contact and Charlie received secondary contamination from her but that remains to be confirmed. The priority now is working out what that item was and where it is now.
This has been slow going for practical reasons. Investigators need to work in heavy hazmat suits in intense heat to go through the various locations looking for an item which might be tiny.
They do not even know what it could look like. It could be some kind of syringe or small glass container but could look like an everyday item like a perfume bottle.
Public Health England said the risk to the general public “remains low” but warned people not to pick up “any strange items such as needles, syringes or unusual containers”.
They have also offered advice for people who may have visited one of five areas identified by police.
Those locations are Muggleton Road, Boots pharmacy and the Baptist church in Amesbury, and John Baker House and Queen Elizabeth Gardens in Salisbury.
Anyone who was there between 22:00 BST on 29 June and 18:30 on 30 June should continue to follow advice, including washing their clothes in a washing machine, keeping other items double-bagged and securely fastened if they are dry-clean only.
Wiltshire Police Chief Constable Kier Pritchard said the news of Ms Sturgess’s death would “send a huge shockwave across our communities” but stressed that “the risk to the wider public remains low”.
He praised residents for their “continual strength and resilience” during “these challenging times”.