Counter-terror police say a couple left in a critical condition in Wiltshire were exposed to the same nerve agent as former Russian spy Sergei Skripal and his daughter Yulia earlier this year.
The pair, named locally as mother Dawn Sturgess and Charlie Rowley, were taken ill on Saturday in Amesbury, around eight miles from where the Skripals were poisoned with Novichok in Salisbury.
It is understood investigators are working on a theory that the pair came into contact with the deadly substance in a part of Salisbury city centre that was outside the clean-up launched after the attack against the Skripals.
A senior Government source told the Press Association it is believed there was cross-contamination of the same batch of nerve agent involved in the "reckless" Salisbury attack, as opposed to a secondary attack.
"They [the authorities] have never been able to ascertain the item used to deposit the Novichok and it's possible the pair have come into contact with that item", the source said.
Test results from the Government's Porton Down laboratory came back at 7pm on Wednesday confirming the substance was Novichok.
The discovery was announced as Downing Street confirmed Home Secretary Sajid Javid will chair a meeting of the Cobra emergency committee on Thursday.
Speaking to reporters from New Scotland Yard, Metropolitan Police Assistant Commissioner Neil Basu said police were investigating links between the two poisonings.
He said: "I appreciate that there will be a great deal of speculation as to whether this incident is linked to the events in Salisbury in March.
"I would add that the complex investigation into the attempted murders of Yulia and Sergei remains ongoing and detectives continue to sift through and assess all the available evidence and are following every possible lead to identify those responsible, for what remains a reckless and barbaric criminal act.
"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. The possibility that these two investigations might be linked is clearly a line of enquiry for us."
Professor Dame Sally Davies, chief medical officer for England, said the Skripal episode meant officials had a "well-established response" in place.
She said: "I understand that those in Salisbury and in surrounding areas will be concerned at this news, particularly those who recently visited areas now cordoned off by police."
She said the risk to the public remains low, but issued "highly precautionary" advice to those with concerns.
Officers were called to a home in Muggleton Road, Amesbury, on Saturday morning when the 44-year-old woman collapsed. They were called back later that day when the man, 45, also fell ill.
Wiltshire Police initially thought that the couple had taken contaminated crack cocaine or heroin, but then decided to carry out further tests and on Wednesday declared a major incident.
Cordons remained in place in areas the pair were known to frequent in Amesbury and in Salisbury, including Queen Elizabeth Gardens and Amesbury Baptist Centre.