A man and woman found unconscious in Wiltshire were poisoned by Novichok, the same nerve agent as ex-Russian spy Sergei Skripal, police say.
The couple, believed to be Charlie Rowley and Dawn Sturgess, are in a critical condition having been found unconscious at a house on Saturday.
Police say no one else has presented with the same symptoms.
There was "nothing in their background" to suggest the pair were targeted, the Met Police said.
Metropolitan Police Assistant Commissioner Neil Basu said it could not be confirmed whether the nerve agent came from the same batch that Mr Skripal, and his daughter Yulia, were exposed to.
However he said the possibility the two investigations were linked was "clearly a line of enquiry".
The Counter Terrorism Policing Network is now leading the investigation, working alongside Wiltshire Police.
The confirmation that the pair from Amesbury, Wiltshire came into contact with Novichok was confirmed following analysis at the defence research facility at Porton Down, Wiltshire.
As a precautionary measure, sites in Amesbury and Salisbury, believed to have been visited by the couple before they fell ill have been cordoned off.
There is no evidence to suggest either visited the sites that were decontaminated following the poisoning of Sergei and Yulia Skripal.