Police are continuing to investigate the "unknown substance" that left a former Russian double agent and his daughter in a critical condition in hospital.
Sergei Skripal, 66, and his 33-year-old daughter, Yulia, were found unconscious on a bench in Salisbury on Sunday.
Several members of emergency services personnel who responded to the call were themselves taken ill, with one remaining in hospital.
Investigators have seized CCTV footage from a nearby gym showing a man and woman, believed to be the victims, walking nearby just half an hour before police were called.
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Boris Johnson, the Foreign Secretary, confirmed the victims' identities while answering an urgent question on the incidnet in the House of Commons.
“It is too early to speculate as to the precise nature of the crime or attempted crime that has taken place in Salisbury, but I know members will have their suspicions and what I will say is that if those suspicions prove to be well-founded than this Government will take whatever measures we deem necessary to protect the lives of people in this country, our values and our freedoms,” he said.
“Though I am not now pointing fingers, I say to governments around the world that no attempt to take innocent life on UK soil will go either unsanctioned or unpunished.”
An Italian chain restaurant has been closed as a precaution, with staff being interviewed as the probe continues.
Relatives told the BBC that Ms Skripal, 33, was in Britain to visit her father. His son is said to have died in non-suspicious circumstances while on holiday in Russia last year.
Mr Skripal is a former Russian double agent, who was convicted in his home country after spying for the UK.
Once a colonel in Russia's GRU military intelligence service, he was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style swap in Vienna.
The case has been compared to that of former FSB officer Alexander Litvinenko, who died after being poisoned with radioactive polonium-210, which was believed to have been put in his tea.
The Independent understands that police are looking for similarities between the cases and the modus operandi used.
Counter-terror police are helping with the investigation, although the incident itself is not terror-related.
Wiltshire Police said that Public Health England “reiterated that, based on the evidence to date, there is no known risk to the public's health.
“However, as a precaution they have advised that if you feel ill contact NHS on 111. If you feel your own or another's health is significantly deteriorating, ring 999”.
A major incident was declared at Salisbury District Hospital after the patients arrived, with people told not to attend A&E unless it is “extremely urgent”.
In Russia, Kremlin spokesman Dmitry Peskov told a conference call with reporters Russia that the government did not “have any information” but was ready to cooperate with Britain if asked.
