Sajid Javid, the UK Home Secretary, has called on Russia to explain the Wiltshire poisonings, as he warned of the risk of Britain's towns becoming "dumping grounds" for nerve agents.
Mr Javid said it was "completely unacceptable for our people to be either deliberate or accidental targets", after a couple were poisoned by the same Novichok used in the Salisbury attack.
Charles Rowley (45) and his 44-year-old girlfriend Dawn Sturgess remain critically ill after being exposed to the substance. They were reportedly left frothing at the mouth and hallucinating.
It was the same type of poison used against Sergei Skripal, the 66-year-old former Russian spy, and his daughter Yulia (33) in a suspected Kremlin-backed assassination attempt in Salisbury in March.
Amid fears the public could still be at risk, UK security minister Ben Wallace earlier confirmed the "working assumption" was the couple taken ill in Amesbury - around 12km from Salisbury - were not targeted victims, but encountered the substance accidentally.
Novichok can be inhaled as a fine powder, absorbed through the skin or ingested.
Mr Javid told the House of Commons: "The use of chemical weapons anywhere is barbaric and inhumane.
"The decision taken by the Russian government to deploy these in Salisbury on March 4 was reckless and callous - there is no plausible alternative explanation to the events in March other than the Russian state was responsible."
Confirming Britain will be consulting with international partners and allies following the latest developments, Mr Javid said: "The eyes of the world are on Russia, not least because of the World Cup.
"It is now time the Russian state comes forward and explains exactly what has gone on."
He accused Moscow of working to undermine UK and international security, saying: "We will stand up to the actions that threaten our security and the security of our partners."
And he warned against a possible Kremlin disinformation campaign, telling MPs: "We have already seen multiple explanations from state-sponsored Russian media regarding this latest incident.
"We can anticipate further disinformation from the Kremlin as we saw following the Salisbury attack."
Mr Javid added: "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."
Earlier, Mr Wallace said Russia could "fill in all the clues to keep people safe". He called on Moscow to provide information, saying: "The Russian state could put this wrong right. They could tell us what happened. What they did. And fill in some of the significant gaps that we are trying to pursue.
"We have said they can come and tell us what happened. I'm waiting for the phone call from the Russian state. The offer is there. They are the ones who could fill in all the clues to keep people safe."
As Mr Rowley and Ms Sturgess remain in comas in hospital, the Kremlin "categorically denied" any Russian involvement in the original Salisbury attack and claimed Britain "showed no interest" in a joint investigation.
But, linking the incident to the attack on the Skripals, Mr Wallace said: "I think what we said at the time was that this was a brazen and reckless attack in the heart of a very peaceful part of the United Kingdom.
"That is part of the anger I feel about the Russian state, is that they chose to use clearly a very, very toxic, highly dangerous weapon."
The Kremlin described the Amesbury poisoning as "disturbing", but said it had not received any appeal from the UK about the incident.
"It triggers profound concern in connection with the similar incidents in the UK," Kremlin spokesman Dmitry Peskov said in Moscow, adding: "We wish them a speedy recovery."
Police have said there is nothing in the couple's background to suggest they had been deliberately targeted.
Investigators are understood to be working on a theory that Mr Rowley and Ms Sturgess came into contact with the deadly substance in a part of Salisbury city centre that was outside the clean-up area.
Authorities will now face questions as to whether the multi-million-pound decontamination effort failed.
Mr Wallace added there was a low risk to the public, stating: "In the whole area of national security, we can't give 100pc guarantees, and we try to minimalise the risk, and I know the people of Salisbury will be anxious. We are giving a low risk, but take some precautions."
Test results from the government's Porton Down laboratory confirmed the substance was Novichok, which was developed by the Soviet Union in the 1970s as a new kind of chemical weapon that would be harder to detect and more potent than existing nerve agents.
People who visited the areas cordoned off in the city over recent days have been warned to wash their clothes and wipe down any items they were carrying at the time.