A man and woman found unconscious in Wiltshire were exposed to Novichok – the same nerve agent that poisoned ex-Russian spy Sergei Skripal, police say.
The couple, believed to be Charlie Rowley, 45, and Dawn Sturgess, 44, fell ill at a house in Amesbury on Saturday and remain in a critical condition.
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.
Home Secretary Sajid Javid will chair a meeting of the government’s emergency Cobra committee later to discuss the developments.
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.
But he said the possibility 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 [the couple’s] movements” in order to determine where they were poisoned.
He added that members of the public should not pick anything up if they didn’t know what it was.
“We have no idea what may have contained the nerve agent at this time,” he said.
In March, Mr Skripal, a former Russian double agent, and Yulia were found slumped on a bench in Salisbury – about 10 miles from Amesbury – after being exposed to Novichok.
The two were critically ill in hospital for weeks but both were discharged in less than three months.
The UK government blamed Russia for the assassination attempt but Russia has denied any involvement and accused the UK of inventing a “fake story”.
Novichok, a resilient and resistant nerve agent developed by the Soviet Union in the 1970s and 1980s, takes effect within minutes, blocking messages from the nerves to the muscles causing bodily functions to collapse.
The highest concentration of it was found on the Skripals’ front door.
A multimillion pound operation took place to decontaminate nine locations in Salisbury.
The Counter Terrorism Policing Network is now leading the investigation, working with Wiltshire Police.
The BBC’s security correspondent Gordon Corera said the most likely hypothesis was that the Novichok was left over from the attack on the Skripals.
He suggested the couple may have come across the nerve agent in a park or a house, which could provide new leads on where it was “brought and put together”.
Chemical weapons expert Richard Guthrie said it was possible that Novichok may have been disposed of “in a haphazard way”.
If the couple had come across it in a syringe or pot, it might have been better preserved, he told BBC Breakfast.
England’s chief medical officer, Sally Davies, said the risk to the general public remained low.
“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,” she said.
Asked whether the people of Salisbury and Amesbury were at risk, security minister Ben Wallace said the “best intelligence officers in the world” were working on the investigation but until the full picture of the Skripal assassination attempt was known, he couldn’t offer complete reassurance.
On Saturday, paramedics were called twice to the property in Amesbury – in the morning, after Ms Sturgess had collapsed, then later the same day, after Mr Rowley had also fallen unwell.
Wiltshire Police said it was initially thought the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs.
The news that Novichok was to blame was announced following analysis at the defence research facility at Porton Down, Wiltshire.
Sites in Amesbury and Salisbury believed to have been visited by the couple before they fell ill have been cordoned off as a precaution, including a church, park and chemist.
On Thursday morning, the focus of police activity was in Muggleton Road where the couple first fell ill, BBC correspondent Jon Kay said.
There is no evidence yet to suggest the couple visited the sites that were decontaminated after the Skripals were poisoned.
On Wednesday evening, before police had made the Novichok link, government-run TV station Rossiya 1 suggested that the Amesbury poisoning may have been staged by the British government out of spite over the “fabulous” World Cup hosted by Russia.
And state TV’s Channel One and Gazprom-Media’s NTV sarcastically argued that it may only be a matter of time before Russia gets the blame.
Home Secretary Sajid Javid said his thoughts were with the two affected and thanked the emergency services and staff at Salisbury District Hospital, where the Skripals were also treated.
He said the events followed “the reckless and barbaric attack” in Salisbury in March.