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”.
Mr Basu said no contaminated items had yet been found but officers were putting together a “very detailed examination of their movements” in order to determine where the pair had been exposed to the substance.
“We [the public] shouldn’t be picking up anything that we have no idea what it is because we have no idea what may have contained the nerve agent at this time,” he added.
The Counter Terrorism Policing Network is now leading the investigation, working with Wiltshire Police.
England’s chief medical officer, Sally Davies, said: “I want to reassure the public that the risk to the general public remains low.”
The Skripal episode meant officials had a “well-established response” in place, she said.
“As before, my advice is to wash your clothes and wipe down any personal items, shoes and bags, with cleansing or baby wipes before disposing of them in the usual way.
“You do not need to seek advice from a health professional unless you are experiencing symptoms, as any individual who had been significantly exposed at the same time would by now have symptoms.”
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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.
Local residents have been warned to expect to see an increased police presence – including officers wearing protective equipment.
Home Secretary Sajid Javid said his thoughts were with the two individuals affected and thanked the emergency services and staff at Salisbury District Hospital.
He said the events follow “the reckless and barbaric attack which took place in Salisbury in March.”
“The government’s first priority is for the safety of the residents in the local area but as Public Health England has made clear, the risk to the general public is low,” he said.
“Tomorrow [Thursday] I will chair a meeting of the government’s emergency committee Cobra in relation to the ongoing investigation.”
Security correspondent Gordon Corera said the poisoning was “hugely significant” as the public “will be worried about public health”.
He also added: “The most likely hypothesis is that this is leftover Novichok from the attack on the Skripals back in March.
“Perhaps this is some of the Novichok prepared for the attack and discarded: maybe somewhere like a park, a house, and maybe these two came across it.”
He added it could give counter-terrorism investigators new leads on where the nerve agent was “brought and put together” before the attack on the Skripals.
Wiltshire pair ‘poisoned by nerve agent’