Cover: Police cordoned off a number of areas in Amesbury (Picture: PA).
Defence experts at a military research site have confirmed another nerve agent poisoning in Wiltshire.
Porton Down, which was previously involved in the testing of the Novichok nerve agent which targeted a former Russian spy and his daughter, has confirmed the use of the same agent again in Wiltshire.
Following the discovery, the Home Secretary Sajid Javid met with the government's emergency Cobra committee.
In a statement to MPs on Thursday, Mr Javid demanded that Russia provide an explanation for the two poisonings, despite the Kremlin denying the accusations.
The Home Secretary accused the Russian state of using Britain as a "dumping ground for poison", following the second Novichok poisoning in four months on UK soil.
He said: "It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks, our towns, to be dumping grounds for poison."
"It is now time that the Russian state comes forward and explains exactly what has gone on."
Mr Javid added the UK will "stand up to the actions that threaten our security".
Shadow Home Secretary Diane Abbott warned that British streets must not be allowed to become "killing fields for state actors".
Police have said there is no evidence that either of the latest victims had recently visited any of the sites that were part of the original clean-up.
There was "nothing in their background" to suggest the pair were targeted, the Met Police said.
Dr Michelle Carlin, senior lecturer in forensic and analytical chemistry at Northumbria University, said it would be "unusual" for the couple in Amesbury to have been affected by the same source.
She said: "It is difficult to speculate whether this is from the original source used in the Skripal case.
"However, if it was from the same source, it is unusual that it has taken four months for someone to be affected by it."
"This is a precautionary measure while we continue to investigate how they came into contact with the substance," he said.
"I do want to reassure the public, however, that there is no evidence that either the man or woman recently visited any of the sites that were decontaminated following the attempted murders of Sergei and Yulia Skripal."
The Department for Environment, Food and Rural Affairs (Defra) handed back four sites to the council so far, the latest a council and police building on 28 June.
Two sites in the city centre remain closed off - The Mill pub and Zizzi, the restaurant where the Skripals dined.