Dawn Sturgess, from Durrington, died at Salisbury District Hospital earlier this evening, the Meteropolitan Police said.
Ms Sturgess' partner Charlie Rowley remains in a critical condition in hospital.
Police said that given the deadly dose, the British couple were believed to have become exposed to Novichok by handling a "contaminated item" - with speculation that it could have been the container used to administer the nerve agent to the Skripals.
Police suspect Rowley and Sturgess handled a discarded item from the first attack, though they have not determined for certain that the two cases are linked. Their investigation must be led by the evidence available and the facts alone.
Police have launched a murder inquiry and a post-mortem examination is due to take place.
Defence Secretary Gavin Williamson blamed Russian Federation for committing "an attack on British soil" over the latest poisonings, some four months after Sergei and Yulia Skripal were targeted. British government officials chose to boycott the games after the Skripal poisonings. In the four months since the Skripals and Nick Bailey were posioned, no other people besides Dawn and Charlie have presented with symptoms.
Britain maintains the March attack on the Skripals had been ordered by the Russian government, a charge denied by representatives of Russian President Vladimir Putin.
The nerve agent was used in the March attack on an ex-Russian spy and his daughter and two Britons have been critically ill since they were exposed to it eight days ago.
Maria Zakharova said: 'This is no longer a laughing matter for MPs in Britain.
Police and public health officials insist the risk to the wider public remains low.
Prime Minister Theresa May said she was "appalled and shocked", and announced a murder investigation.
More than 100 police are trying to search all areas where Sturgess and Rowley had been before they became ill.
But it advised against picking up "any odd items such as needles, syringes or unusual containers".
The statements from Russian diplomats came in response to the accusations produced by the UK Defense Secretary Gavin Williamson, who pinned the blame for the demise of Sturgess squarely on Moscow and called on the world to "unite" against the Kremlin... again.
A police officer stands at a cordon around a public litter bin next to a supported housing project in Salisbury after a major incident was declared when a man and woman were exposed to the Novichok nerve agent on July 5, 2018 in Salisbury, England.
The hospital's medical director, Dr. Christine Blanshard, said the staff "worked tirelessly to save Dawn".