The British Home Office accused Russia of "reckless and callous" behavior and demanded that Moscow provide details about the nerve-agent attack on a Russian former double agent and his daughter in March after two British citizens were poisoned with the same substance last week.
"The eyes of the world are currently on Russia," Home Secretary Sajid Javid told lawmakers on July 5, while urging Russia to explain "exactly what has gone on" in Salisbury.
"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 said.
The comments ramping up pressure on Moscow came as a man and a woman were hospitalized in critical condition after falling ill at a house in the English town of Amesbury on June 30.
The town is just 11 kilometers away from Salisbury, where ex-Russian spy Sergei Skripal and his daughter, Yulia, were poisoned in March.
Britain blames Russia for the assassination attempt, but Moscow denies any involvement.
Javid told parliament after chairing a meeting of the government's Cobra emergency committee that a link between the Skripal case in Salisbury and the June 30 poisonings was the "main line of inquiry" for police in the case.
Earlier on July 5, Security Minister Ben Wallace told the BBC that "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."
"They are the ones who could fill in all the clues to keep people safe," he added.
Calling the new incident "very worrying," the Kremlin later said it was unaware of any official British requests to Russia for help in relation to the investigation.
Kremlin spokesman Dmitry Peskov also said that Moscow had offered Britain its assistance in probing the Salisbury attack long ago but had been rebuffed.
The Russian Foreign Ministry on July 5 said the British government should end "dirty political games" and "intrigues" over the two poisoning incidents and agree to work with Moscow on the probe.
"Right now, it is important to launch a full-scale investigation," spokeswoman Maria Zakharova said.
"Our law enforcement agencies are ready for this job. We inform the British side about this regularly, several times a month, via diplomatic channels," she added.
Late on July 4, Britain's top counterterrorism officer, Neil Basu, said tests had found that the two Amesbury residents had been exposed to Novichok, the same nerve agent that sickened the Skripals.
Basu said they could not confirm whether the nerve agent came from the same batch but that the possibility was "clearly a line of inquiry."
It was unclear how the 45-year-old man and the 44-year-old woman came into contact with the nerve agent, and there was "nothing in their background" to suggest the pair were targeted, the official said.