The UK's home secretary has demanded an explanation from Russia after two people were exposed to the military-grade nerve agent Novichok in Amesbury, Wiltshire, on Saturday.
The incident, which has left a man and woman both in their forties critically ill, comes less than four months after the UK accused the Kremlin of poisoning former Russian military spy Sergei Skripal and his daughter, Yulia, with the same substance in nearby Salisbury.
Speaking in the House of Commons on Thursday after a meeting of the government's emergency Cobra committee, UK home secretary Sajid Javid said that any links between the two incidents are "the main line of enquiry".
Javid stopped short of blaming Moscow overtly, saying it would be wrong to "jump to conclusions" while the investigation is carried out. However, he said that the UK was "ready to respond as and when new evidence comes to light".
"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," he said.
"There is no plausible alternative explanation to the events in March other than that the Russian state was responsible."
Police said later in the day that the victims are understood to have touched a contaminated item.
“Following further tests of samples from the patients, we now know that they were exposed to the nerve agent after handling a contaminated item,” the Met Police said in a statement.
Sajid Javid noted in his address to MPs that Russian state-sponsored media has already begun to spread "disinformation" since news of the latest incident was announced on Wednesday.
The Russian embassy in the Netherlands wrote on Twitter that the UK must think Russia is "dumb" to carry out such an attack "again", adding: "The show must go on."
Kier Pritchard, chief constable of Wiltshire police, told a press conference in Salisbury: "As we’ve previously stated we were contacted by the South West Ambulance Service following the admission of the second individual to Salisbury hospital on Saturday afternoon.
"Based upon items that had been located the address there was a strong indication that the decline in their condition was related to a batch of illegal drugs.
"Therefore our response based on this hypothesis was entirely proportional and I stand by the actions of our officers.
"Samples were sent to DSTL [Defence Science and Technology Lab] at Porton Down. Preliminary results of these tests came back on Tuesday evening. This obviously shifted our working hypothesis. We then declared a major incident."
He added that the possibility that this investigation would be linked to the Skripal case was "a line of enquiry" but he cautioned against speculation.
Speaking earlier on the Today programme, minister of state for security Ben Wallace said the government is working on the assumption that, unlike the Skripals, the latest victims were not actively targeted. Yet Javid said he "cannot rule out" that the Novichok found in both cases came from the same batch.
"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," 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."
Prime minister Theresa May also harked back to the Skripal poisoning on Thursday, which she described as a "brazen and reckless" murder attempt.
"Once again the public is having to contend with the consequences of two people being exposed to a nerve agent and I would like to personally thank local businesses and residents for their cooperation," she said.
However, Javid said there is "no evidence" that the latest victims – named as Charlie Rowley, 45, and Dawn Sturgess, 44 – visited any of the same places as the Skripals had before they were poisoned, which, he said, were made safe after decontamination.
"Our strong working assumption is that the couple came into contact with the nerve agent in a different location to sites which had been part of the original clean up operation," he said.