Charlie Rowley, 45, and Dawn Sturgess, 44, of Amesbury, were named by friends as the duo who fell ill after being exposed to the same nerve agent that nearly ended the lives of Seregi and Yulia Skripal on March 4.
Flanked by the chief medical officer Dame Sally Davies, and amid speculation the British couple had been struck down by sites still contaminated after the attempted assassination of the Skripals, Basu said: "I do want to reassure the public, however, that there is no evidence that either the man or woman recently visited any of the sites that were decontaminated following the attempted murders of Sergei and Yulia Skripal".
Kremlin spokesman Dmitry Peskov described the case as "very worrying" but said Russian Federation had no information "about what substances were used and how they were used".
"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".
After several months in the hospital, both Sergei and Yulia have recovered, but many questions about the perpetrators and their motives remain. But British security officials have said the latest case likely was not the result of direct targeting, and rather a ripple effect from the first incident.
Sam Hobson, a 29-year-old friend of the couple, names them as Dawn Sturgess and Charlie Rowley, a couple, and describes how he had been with them enjoying the sunshine the previous day.
Wiltshire Police initially thought the pair had fallen ill after using a contaminated batch of heroin or crack cocaine.
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Investigators believe Mr Rowley and Mrs Sturgess had spent Friday evening in Salisbury, visiting Queen Elizabeth gardens - a park about 500 yards away from The Maltings, where the Skripals were found collapsed on March 4. The pair are being treated at Salisbury District Hospital.
"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".
Meanwhile, UK police say they are investigating whether there is a link between the two cases.
Speaking to BBC security correspondent Frank Gardner, the source said Novichok could be degraded by rainwater and sunlight over time - meaning it was probably discovered by the couple in a contained space. They may return, police said.
"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 we are trying to pursue".
Moscow hit back by expelling Western diplomats, questioning how Britain could know that Russian Federation was responsible and offering rival interpretations, including that it amounted to a plot by British secret services.