The home secretary has accused Russia of using Britain as a "dumping ground for poison" after a second incident involving the nerve agent Novichok.
Charlie Rowley, 45, and Dawn Sturgess, 44, collapsed at a house in Amesbury, Wiltshire, on Saturday and remain critically ill.
Sajid Javid said the 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.
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 the nerve agent 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.
"We cannot attribute this to the same batch at this point and scientists will be looking into that," he said.
"I'm also told that may not even be possible because of a number of factors, but we cannot rule out, of course, that it was from the same batch."
BBC security correspondent Gordon Corera said the most likely hypothesis was that the Novichok was left over from the attack on the Skripals.
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".