British counterterrorism experts are working with police, but this incident has not been declared as a terror attack.
Investigators are seeking clues after the two were exposed to the Soviet military-grade substance "Novichok".
They first responded to a woman, 44, who had lost consciousness, and later in the day to a man, 45, who began exhibiting unusual behavior, a friend who was with him told authorities.
According to law enforcers, on the evening of June 30, a 44-year-old woman and a 45-year-old man who lived together were found unconscious in their home.
The hospital is where the Skripals also spent weeks in a critical condition before slowly recovering and being discharged. Forensic searches were due to be carried out at the Amesbury home where Ms. Sturgess and Mr. Rowley collapsed, and other sites the couple visited before they were sickened.
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Officials said the highest concentration of Novichok was found on the couple's hands, leading them to suspect they encountered the poison from handling some contaminated item. "We urge British law enforcement not to get involved in dirty political games", foreign ministry spokeswoman Maria Zakharova told reporters.More news: United States senators on rare visit to Moscow ahead of Putin-Trump summit
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"This government and its representatives will have to apologise to Russian Federation and the worldwide community", she said, adding: "It will happen". Moscow has denied involvement.
The British woman who was recently exposed to the same nerve agent that nearly killed a Russian spy and his daughter earlier this year has died, police said Sunday. In a televised statement in the House of Commons, he added that the latest couple is thought to have been exposed to the nerve agent at a location not previously associated with the Skripals incident. "We can not attribute this to the same batch at this point".
Britain blames Russian Federation for the Skripals' poisoning.
This latest case of poisoning has raised public health concerns in the Salisbury area, where a massive decontamination effort took place after the Skripals were poisoned.
"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", Mr. Javid said.
Meanwhile, Russian lawmaker Sergei Zheleznyak has described the incident as part of British efforts to tarnish the Russia-hosted World Cup and fuel tensions ahead of a planned US-Russia summit.
The British authorities say there was nothing in the background of the latest pair to suggest a link to espionage or to Russian Federation.