The British Government will hold an emergency COBRA meeting today
UK counter-terrorism police have confirmed that two people found critically ill near Salisbury were exposed to the nerve agent novichok.
It is the same Soviet-era agent used against former Russian spy Sergei Skripal and his daughter in Salisbury in March.
The pair, both aged in their 40s, fell ill at a house in Amesbury, Wiltshire on Saturday.
Officers were called to the house when a woman - named locally as 44-year-old Dawn Sturgess - collapsed and were called back later that day when the man - named locally as 45-year-old Charlie Rowley - also fell ill.
The paramedics who took Mr Rowley away on Saturday were wearing hazardous material protective suits.
They are both in critical condition at Salisbury District Hospital.
Police initially believed the pair became unwell after using heroin or crack cocaine from a contaminated batch of drugs.
Speaking at a news conference last night the Met Police Assistant Commissioner for Counter-Terrorism Neil Basu said there had been a "significant development" after officers declared a "major incident" earlier in the day.
"This evening we have received test results from Porton Down that show the two people have been exposed to the nerve agent novichok," said Mr Basu.
The incident took place roughly eight miles (13km) from Salisbury, where former Russian spy Sergei Skripal and his daughter Yulia were targeted with the nerve agent novichok in March.
Police are investigating whether the cases are linked - but have said there is "nothing in their background" to suggest the pair were targeted.
Assistant Commissioner Basu said both patients are British nationals and are local to the area.
Police are currently working to identify their next of kin.
He said there are 100 Scotland Yard detectives working with officers in Wiltshire and warned of an "increased police presence" in the area.
He said officers will be wearing protective clothing as a "precaution" at several cordoned off sites in Amesbury and Salisbury.
"I must say that we are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to,” he said.
“The possibility that these two investigations might be linked is clearly a line of enquiry for us.
"It is important, however, that the investigation is led by the evidence available and the facts alone and we don't make any assumptions."
When asked if the pair were deliberately targeted, Mr Basu said: "That is a theory but it's speculation at the moment. We don't have any intelligence or evidence that they were targeted in any way. There is nothing in their background to suggest that at all."
Sally Davies, chief medical officer for England, said that risk to the public "remains low" and said anyone exposed at the same time would have by now witnessed symptoms.
She issued "highly precautionary" advice to those concerned and 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."
In a statement Wiltshire's Chief Constable Kier Pritchard said: "We cannot underestimate the impact the shocking news of a second major incident in this part of our county in such a short space of time will have.
"The communities in Salisbury and Amesbury have shown extraordinary resilience and spirit since the events of March 4 and I have no doubt they will rise to this latest challenge in the same way."
He added: "I would urge anyone with concerns to speak to our officers on the cordons or to contact the dedicated phone number which has now been set up.
Downing Street has since confirmed Home Secretary Sajid Javid will chair a Cobra meeting on Thursday.