The Home Secretary has accused the Russian state of using Britain as a "dumping ground for poison" after a second nerve agent emergency in four months.
In a blistering attack, Sajid Javid demanded that the Kremlin provide an explanation for the two episodes, which investigators believe may be linked.
His remarks come as Dawn Sturgess and Charlie Rowley fight for their lives in hospital after they were exposed to the chemical weapon Novichok.
The couple were taken ill on Saturday in Amesbury, around eight miles (13km) from where former Russian spy Sergei Skripal and his daughter Yulia were poisoned with the same agent in Salisbury in March.
One theory understood to be under investigation is that the pair who were poisoned in the latest incident may have inadvertently found a container - such as a phial or syringe - used to transport the nerve agent for the initial attack on the Skripals and discarded in a public place.
Novichok remains highly toxic for a considerable period of time, so even the tiniest trace remaining in a container picked up by the victims could account for their severe illness.
In a statement to the House of Commons, Mr Javid said: "The eyes of the world are currently on Russia, not least because of the World Cup.
"It is now time that the Russian state comes forward and explains exactly what has gone on."
Making clear that the UK will "stand up to the actions that threaten our security", he 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."
Officers were called to a home in Muggleton Road, Amesbury, on Saturday morning when 44-year-old woman Ms Sturgess collapsed.
They were called back later that day when Mr Rowley, 45, also fell ill.
It was initially believed that the two patients had possibly been using drugs from a contaminated batch, police said.
But after further tests, authorities declared a major incident and on Wednesday night counter-terror police assumed responsibility for the investigation after the Government's Porton Down laboratory concluded that the pair had been exposed to Novichok.
Mr Javid told MPs that he "cannot rule out" the possibility that the Novichok found in Amesbury was from the same batch used in the Salisbury attack.
Amid questions about the post-Salisbury clean-up operation, the Home Secretary said the risk to the public remained low.
He added: "We have taken a very robust approach to decontamination and there is no evidence that either the man or the woman in hospital visited any of the places that were visited by the Skripals.
"Our strong working assumption is that the couple came into contact with the nerve agent in a different location to the sites which have been part of the original clean-up operation."
As the fall-out continued:
- Shadow home secretary Diane Abbott warned that British streets must not be allowed to become "killing fields for state actors".
- Prime Minister Theresa May declared that Salisbury is "very much open for business" after residents raised fears that the new case will hamper the local economy.
- Six sites visited by Mr Rowley and Ms Sturgess before they fell ill were cordoned off.
- The Kremlin described the Amesbury poisoning as "disturbing" and restated Russia's denial of involvement in the Salisbury attack.
- More than 100 counter-terrorism detectives were assigned to a huge investigation to establish how the pair were exposed to the nerve agent, and determine whether there is any firm link to the Salisbury attack.
- The UK reported the incident to the Organisation for the Prohibition of Chemical Weapons.
Earlier on Thursday, Security Minister Ben Wallace said the latest victims were not directly targeted.
He told BBC Radio 4's Today programme: "The working assumption would be that these are victims of either the consequence of the previous attack, or something else."
The episode in Salisbury - the first use of a nerve agent in Europe since the Second World War - sparked international outrage.
Mr Skripal and his daughter have since left hospital.