Police have launched a murder inquiry after a woman exposed to the nerve agent Novichok in Wiltshire died.
Dawn Sturgess, 44, died in hospital on Sunday evening after falling ill on 30 June.
Charlie Rowley, 45, who was also exposed to the nerve agent in Amesbury, remains critically ill in hospital.
Kremlin spokesman Dmitry Peskov said Russia was "deeply concerned" about the incident - but said accusations that it was involved are "absurd".
The UK government previously blamed Russia for the poisoning of former spy Sergei Skripal and his daughter Yulia with Novichok in Salisbury in March. Russian authorities denied any involvement in the poisoning.
Investigators say the possibility that the two cases are linked is a "clear line of inquiry".
Theresa May said she was "appalled and shocked" by the death of mother of three Ms Sturgess, from Durrington.
Home Secretary Sajid Javid will chair a meeting of the government's emergency committee Cobra later today.
A post-mortem examination of Ms Sturgess is due to take place and her family has been informed, police said.
The investigation is being led by detectives from the Counter-Terrorism Policing Network and about 100 detectives are working on the case alongside Wiltshire Police.
Officers are still trying to work out how Ms Sturgess and Mr Rowley were exposed to the nerve agent - although tests have confirmed they touched a contaminated item with their hands.
Assistant Commissioner Neil Basu, the head of UK Counter-Terrorism policing, said: "This is 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.
"The 45-year-old man who fell ill with Dawn remains critically ill in hospital and our thoughts are with him and his family as well."
Mr Basu said the death "has only served to strengthen our resolve to identify and bring to justice the person or persons responsible for what I can only describe as an outrageous, reckless and barbaric act".
He said: "Detectives will continue with their painstaking and meticulous work to gather all the available evidence so that we can understand how two citizens came to be exposed with such a deadly substance that tragically cost Dawn her life."
Dr Christine Blanshard, medical director at Salisbury District Hospital, where Ms Sturgess died, said it was "desperately sad" and she expressed condolences to her family.
She said: "The staff here at Salisbury District Hospital worked tirelessly to save Dawn. Our staff are talented, dedicated and professional and I know today they will be hurting too.
"They did everything they could."
Mr Rowley is also being treated at the hospital.
A police officer who was tested for Novichok poisoning was given the all clear.
Mrs May sent her "thoughts and condolences" and said officials were "working urgently to establish the facts".
She said: "The government is committed to providing full support to the local community as it deals with this tragedy."
British diplomat Julian King, the European Commissioner responsible for the EU's security union, said: "Those behind this are murderers."
Home Secretary Sajid Javid said: "This desperately sad news only strengthens our resolve to find out exactly what has happened."
Labour leader Jeremy Corbyn said he was "shocked", adding: "A full and thorough police investigation must now establish the facts, provide support to the local community and bring those responsible to justice."
Public Health England said the risk to the general public "remains low".
Professor Paul Cosford said: "As a precaution we still advise the public not to pick up any strange items such as needles, syringes or unusual containers."
He also said people in five locations between 22:00 BST on 29 June and 18:30 on 30 June should continue to follow advice, namely: "Wash your clothes in a washing machine and to keep your items double-bagged and securely fastened, if they are dry-clean only."
Those locations are Muggleton Road, Boots pharmacy and the Baptist church in Amesbury, and John Baker House and Queen Elizabeth Gardens in Salisbury.
Baroness Scott, the leader of Wiltshire Council, said the residents of John Baker House - the hostel where Ms Sturgess lives - have been rehoused and are being given support.
Angus Macpherson, Wiltshire police and crime commissioner, said he was "horrified and appalled".
He said: "Ms Sturgess was an innocent member of the public who should have been able to go about her daily life without becoming an unwilling victim in such an unprecedented, international, incident."
By BBC security correspondent Frank Gardner
The working hypothesis is that the pair became contaminated after touching a poison container left over from the March attack on Sergei and Yulia Skripal.
The death of Dawn Sturgess, a British citizen on British soil, now changes the investigation to a murder inquiry, with all the diplomatic and security ramifications that carries.
Britain has been blaming Moscow for the original attack in March, saying there is no plausible alternative to the Kremlin having ordered the assassination attempt.
Russia has denied any involvement, suggesting instead this was the action of a weak British government looking to undermine the success of the current World Cup being hosted by Russia.
The next few days are likely to see further accusations and counter-accusations.
The poisoning of the Skripals, both of whom spent weeks in hospital before being discharged, was blamed by the UK government on Russia.
Russian authorities denied any involvement.
Mr Skripal, 66, is a Russian former double agent. He was accused by Russia of spying for Britain's MI6 and jailed in 2006. He was later pardoned and allowed to settle in the UK.
After the hospitalisation of Ms Sturgess and Mr Rowley, Mr Javid accused the Russian state of using Britain as a "dumping ground for poison".
The Russian Embassy hit back, accusing the government of trying to "muddy the waters" and "frighten its own citizens".
Russian media has suggested alternative explanations of what happened in Amesbury, including claims the UK government staged it because it is finding the World Cup "hard to swallow" or to disrupt the Russia-US summit on 16 July.
According to BBC Monitoring, other theories in the media include the claim that Porton Down or a former employee of the research lab are to blame, or the Amesbury couple poisoned themselves.
In a statement, the Met Police said the possibility the poisoning of the Skripals and Ms Sturgess and Mr Rowley are linked is a "clear line of inquiry".
A spokesman said the investigators are "not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to".
He also said: "There is no evidence that (Ms Sturgess and Mr Rowley) visited any of the sites that were decontaminated following the attempted murders of Sergei and Yulia Skripal in March."