Britain has demanded answers from Russia after a couple was exposed to the same nerve agent used on a former Russian spy and his daughter in an attempted murder blamed on Moscow.
But Russia quickly hit back, denouncing Britain for playing "dirty political games" and demanding London apologise.
The British couple fell ill on Saturday in Amesbury, a small town near the southwestern English city of Salisbury where former double agent Sergei Skripal and his daughter Yulia collapsed on March 4.
That incident triggered a major diplomatic crisis with Russia after Britain and its allies accused Moscow of trying to kill the Skripals - a charge strongly denied by the Kremlin.
Novichok is a military-grade nerve agent developed by the Soviet Union during the Cold War.
Speaking to parliament on Thursday, Interior Minister Sajid Javid said a link between the cases was "clearly the main line of inquiry".
"It is now time that the Russian state comes forward and explains exactly what has gone on," he said.
"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."
The remarks sparked a sharp response from Moscow.
"We urge British law enforcement not to get involved in dirty political games," foreign ministry spokeswoman Maria Zakharova told reporters.
"This government and its representatives will have to apologise to Russia and the international community," she said.
Drug paraphernalia found
Police said tests on the couple, named locally as Dawn Sturgess, 44, and Charlie Rowley, 45, revealed they were exposed to Novichok, later confirming they had handled "a contaminated item".
It was not immediately clear what the item was, nor if the substance was from the same batch used on the Skripals.
Sturgess collapsed first, Rowley hours later.
Police initially suspected that the couple had consumed a contaminated batch of illegal drugs, saying they had found "paraphernalia" in the house.
A friend of Rowley's told AFP that he was a drug user and Sturgess lived in a homeless hostel in Salisbury.
England's chief medical officer Sally Davies said the public should "be careful of picking up any unknown or already dangerous objects such as needles and syringes".
Kremlin spokesman Dmitry Peskov described the case as "very worrying" but said Russia had no information "about what substances were used and how they were used".
'We cannot detect it'
Around 100 counter-terror officers are working alongside police on the investigation, and several sites in Salisbury and Amesbury that were visited by the couple have been cordoned off, including a park and Sturgess's homeless hostel in Salisbury, as well as a pharmacy, church and the house in Amesbury.
Officials said there was only a "low risk" to the wider public, but urged anyone who had visited the affected sites wash their clothes and wipe down personal items.
Chemical weapons expert Hamish de Bretton-Gordon told AFP the couple likely came into contact accidentally with Novichok residue that may have been discarded.
"The whole search is hugely challenging," he added. "Novichok's designed by the Russians to overmatch NATO chemical defence capabilities, we cannot detect it."
The Novichok used in March was believed to have been smeared on the Skripal's front door.
Police said there was no evidence the latest victims visited any of the sites linked to the Skripals, which have since been decontaminated.
'In another world'
Sam Hobson, a friend of the couple, said he had visited Salisbury with them the day before they fell ill.
Hobson said he went to Rowley's house on Saturday as Sturgess was being taken to hospital and stayed with Rowley for several hours until he too began to complain of feeling ill.
"He was sweating loads, dribbling, and you couldn't speak to him," Hobson said.
"There was no response from him, he didn't even know I was there. It's like he was in another world, hallucinating."
The victim's brother, Matthew Rowley, told ITV News the Salisbury incidents were "outrageous" and said he had not received any information about his brother's condition.
Cara Charles-Barks, the chief executive of Salisbury District Hospital, said both of the patients remained in a critical condition.
"Having been exposed to a nerve agent, they are clearly acutely unwell," she added.
