The couple poisoned by Novichok were exposed to the substance after handling a contaminated item, police say.
Poisoning victims Dawn Sturgess and Charlie Rowley. Photo: Facebook
Charlie Rowley, 45, and Dawn Sturgess, 44, collapsed at a house in Amesbury, Wiltshire, on Saturday and remain critically ill.
Home secretary Sajid Javid said the nerve agent was the same as that used on ex-Russian spy Sergei Skripal and his daughter in March.
Russia said Theresa May's government was subjecting them "to hell".
Russian foreign ministry spokeswoman Maria Zakharova urged police not to be led by the "dirty political game" and said she was confident London would have to apologise to Russia.
Mr Javid accused Russia of using Britain as a "dumping ground for poison" after the second incident involving the nerve agent.
BBC security correspondent Gordon Corera said that because the highest concentration of Novichok was found on the couple's hands, police believe the item they handled could be a container or reciprocal that was used to carry the nerve agent.
He said the most likely hypothesis is that the Novichok was left over from an item discarded after the attack on the Skripals.
The BBC's home affairs correspondent June Kelly said it has been suggested the item "could be a vial or syringe because of the couple's lifestyle".
Debbie Stark, south west deputy director from Public Health England said the risk to the public of further poisoning incidents "remains low".
In a statement to MPs, Mr Javid said: "It is now time that the Russian state comes forward and explains exactly what has gone on."
He said the "strong working assumption" was that the couple came into contact with Novichok in a different location to the sites which had been part of the clean-up operation in near-by Salisbury after the Skripal poisoning.
"It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks, our towns, to be dumping grounds for poison," he added.
He said he could not rule out the possibility that the Novichok found in Amesbury was from the same batch used in the Salisbury attack and that scientists "will be looking into that."
A police officer near the police line in Queen Elizabeth Gardens in Salisbury. Photo: AFP
Earlier, Mrs May said it was "deeply disturbing" to see two more people exposed to Novichok in the UK, and the police would leave "no stone unturned in their investigation".
Shadow home secretary Diane Abbott said British streets must not be allowed to become "killing fields for state actors".
On Saturday, paramedics were called twice to a house in Muggleton Road in Amesbury - first at 11:00 BST after Ms Sturgess collapsed, then later, after Mr Rowley also fell ill.
Wiltshire Police said it was initially thought the two patients had been using heroin or crack cocaine from a contaminated batch of drugs.
They explained that this assessment was based on items located at the address where the couple fell ill, information from a friend of the pair and the "initial medical diagnosis".
In the early hours of Wednesday police declared a "major incident" after revealing the couple had been exposed to an "unknown substance".
By late that evening, Scotland Yard said Novichok was to blame following analysis at the defence research facility at Porton Down, Wiltshire.
Salisbury District Hospital's chief executive Cara Charles-Barks said the couple are "extremely unwell" and remain in a critical condition.
Partners Charlie Rowley and Dawn Sturgess spent Friday afternoon and evening visiting shops in Salisbury, the city where Mr Skripal and his daughter were poisoned.
The following day Ms Sturgess, who is understood to be a mother of three, was taken away on a stretcher by an ambulance crew.
Her friend Sam Hobson said she had appeared to have a fit and was "foaming at the mouth".
Shortly after, Mr Rowley started "rocking against the wall", said Mr Hobson.
"His eyes were wide open, glazed and pinpricked, and he was sweating, dribbling and making weird noises."
Mr Hobson said the police and firefighters were in "hazard suits" and "cordoned it all off".
A police officer stands guard outside a residential address in Amesbury, a man and woman were found unconscious. Photo: AFP
Ms Sturgess' father Stephen, 65, told The Times the family found out from TV news that she may have been poisoned by a nerve agent.
"We heard from the hospital on Sunday, and we gave them Dawn's GP details but we didn't hear anything from the police.
"We know as much as the next person in the street. We actually called the police station for information."
Natalie Smyth from Amesbury said: "I'm furious that it took four days for them [police] to even mention it could have been Novichock."
"Were they lying to us about it being drug related?"
The most likely hypothesis was that the Novichok was left over from the attack on the Skripals, says BBC security correspondent Gordon Corera.
Mr Javid said this was the "leading line of inquiry".
Metropolitan Police Assistant Commissioner Neil Basu said they could not confirm whether the nerve agent came from the same batch but the possibility was "clearly a line of inquiry".
There was nothing in the couple's background to suggest the pair had been targeted, he said.
His officers are examining the couple's movements to determine where they were poisoned.
So far no contaminated items have been found and the police say they have no idea what the nerve agent was contained in.
Chemical weapons expert Richard Guthrie said it was possible that the couple came across the Novichok which poisoned the Skripals after it had been disposed of "in a haphazard way".
- BBC