"Dawn will always be remembered by us as a gentle soul who was generous to a fault", they said in a statement released by London's Metropolitan Police.
Lorna Wilkinson, Salisbury District Hospital's director of nursing, said Mr Rowley's condition was now serious, but stable.
Police have yet to establish how the couple, 44-year-old Dawn Sturgess and 45-year-old Charlie Rowley, came into contact with the nerve agent.
Assistant Commissioner Neil Basu, Britain's top anti-terrorism police officer, said the death "has only served to strengthen our resolve" to find those responsible.
"Our hypothesis is that they must have handled a container we are now seeking", he said.
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Residents of the homeless hostel in Salisbury where Sturgess lived, which was evacuated after the couple fell ill, expressed their devastation at the news of her death.
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On Sunday, police issued a statement saying there had been no evidence that either Rowley or Sturgess had been to any decontaminated sites from the nerve agent attack on Skripal and his daughter.
A Wiltshire Police spokesman said: "We have arranged the transportation of a vehicle from an address in Swindon this evening in relation to the on-going incident in Amesbury".
"However, if there is anyone who saw them or met with them who we haven't already spoken to, I would urge them to contact police on 0800 789 321".
Basu also said he did not believe there was much novichok in Salisbury - because the attackers would have been "foolish" to carry a large amount in.
More than 100 police officers have been working to locate a small vial or other container thought to have held the nerve agent that sickened the two.
"This is a very rare substance banned by the global community and for there to be two separate, distinct incidents in one small English county is implausible to say the least", Basu added. Sturgess and Rowley were found at a house in Amesbury, 11 km (7 miles) away.
"The brutal reality, however, is that I can not offer you any such assurances or guarantees at this time". It is not known if he has been told that his girlfriend has died.
A member of the audience suggested that authorities had not been looking for the Novichok until the latest incident.
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