A general view shows the main entrance to Salisbury District Hospital in Salisbury, southern England, on March 6, 2018 where a man and a woman remain in critically condition which sparked an ongoing major incident which started on March 4. He had been living quietly in Salisbury, a cathedral city 90 miles (145 kilometers) southwest of London, when he was struck down along with his 33-year-old daughter Yulia, who was visiting him.
Britain accuses Russian Federation of poisoning the Skripals with a Novichok nerve agent, a group of chemical weapons developed by the Soviet Union during the Cold War.
United Kingdom counter-terrorism police are now leading the investigation, though Basu said it was unclear how the two people came into contact with the nerve agent or whether they had been specifically targeted.
The couple left fighting for their lives were earlier named as Charlie Rowley and Dawn Sturgess.
Friends said the couple were foaming at the mouth and had trouble breathing before they were hospitalised.
London's Metropolitan Police said that "given the recent events in Salisbury, " counterterrorism officers were working with local police on the investigation.
The poisoning of two Britons with the Novichok nerve agent looks like an unfortunate after-effect of the attempted murder in March of a former Russian agent and his daughter, Britain's Health Minister, Jeremy Hunt, says.
Dame Sally Davies, the chief medical officer, and Public Health England will come under pressure after insisting the risk to other members of the public was low.
Both Sergei Skripal and his daughter Yulia have since been discharged from hospital.
"It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs", according to the police statement.
Russia, which is now hosting the soccer World Cup, has denied any involvement in the March incident and suggested Britain had carried out the attack to stoke anti-Moscow hysteria.
Paramedics in hazmat suits stretchered them out of an apartment in Amesbury, about eight miles from Salisbury, on Saturday evening.
"At this stage it is not yet clear if a crime has been committed", said Mills, who took the rare step of calling it a major incident.
It is unclear how the two Britons, whose background has nothing to suggest a link to the world of espionage or the former Soviet Union, came into contact with the poison, which is slow to decompose.
Prime Minister Theresa May has been updated on the situation, her office told CNN.
A number of areas in Salisbury and Amesbury remained sealed off as part of the investigation and Mr Pritchard says he understand that will cause concern and disruption.
As a precaution he said more police would be stationed on the streets.
Scientists there concluded a nerve agent was responsible, the same type used to attack the Skripals.
Britain blamed the poisoning of the Skripals on Russian Federation, prompting a serious diplomatic crisis between Moscow and Western states.