Two people were found unconscious on Saturday and are in critical condition in hospital
Counter-terror police in Britain say two people were exposed to nerve agent novichok, which left them in a critical condition.
It is the same agent that was used against former Russian spy Sergei Skripal and his daughter in Salisbury in March.
The pair, both aged in their 40s, were found unconscious at a house in Amesbury, Wiltshire on Saturday night.
They are both in critical condition at Salisbury District Hospital.
The victims have been named locally as Charlie Rowley (45) and Dawn Sturgess (44).
During a news conference earlier, police confirmed both are local people and British nationals.
Sam Hobson, a friend of the pair, said: "I came around Charlie's house in the morning and there was loads of ambulances outside and his girlfriend was getting taken away and he said she was complaining of a headache in the morning.
"She went into the bathroom to have a bath and he heard a thump and she was in there having a fit, foam coming out of her mouth."
He added: "He [Mr Rowley] was sweating loads dribbling and you couldn't speak to him. He was making funny noises and he was rocking backwards and forwards. There was no response for me, he didn't even know I was there, he was in another world - he was hallucinating."
Samples of the substance were sent to experts at the nearby government chemical weapons research laboratory at Porton Down.
Metropolitan Police Assistant Commissioner Neil Basu confirmed: "This evening I have received test results from Porton Down which show that the two people have been exposed to the nerve agent Novichok".
Police say no-one else has presented with the same symptoms linked to this incident.
Around 100 detectives from the Counter Terrorism Policing Network are now working on the investigation, alongside colleagues from Wiltshire Police.
A number of sites in the Amesbury and Salisbury areas have been cordoned off, where police believe the two individuals visited before they fell ill.
The town of Amesbury is roughly eight miles from the city of Salisbury, where the Skripals were found unresponsive on a public bench on March 4th.
The pair each spent weeks in a coma in hospital; however they have both since been discharged.