It later emerged that they had been poisoned by a nerve agent sparking a large-scale investigation that has widened to include a number of sites around the centre of the city, a auto belonging to Skripal, and the graves of his wife and son, Alexandr.
Only three - Sergei Skripal, his daughter Yulia and a police officer - remain in hospital.
"The public should not be alarmed", counter-terrorism police, who are leading the investigation, said in a statement.
Suspicion is mounting that Russian Federation attempted to kill the former intelligence officer and his daughter as an act of revenge against Mr Skripal, who was convicted in 2006 of selling state secrets to MI6. The 66-year-old former agent had been living in Salisbury, 90 miles southwest of London.
Defence Secretary Gavin Williamson said: "Our armed forces have stepped up to support the police in their investigation in Salisbury, building on the vital expertise and information already provided by our world-renowned scientists from the Defence Science and Technology Laboratory at Porton Down".
The Home Secretary is also visiting the hospital which has been treating the victims.
Britain will respond appropriately if evidence shows Moscow sponsored a nerve agent attack on a Russian ex-spy and his daughter in southern England, Prime Minister Theresa May says in her strongest warning to date.
Police have not revealed which nerve agent was involved.
Detectives are retracing the Skripals' movements as they try to discover how and where the toxin was administered.
There was also a flurry of activity at the cemetery in Salisbury where Skripal's wife and son are buried, with forensic teams active in several parts of the site. Skripal's wife, Liudmila, died of cancer at 60, in 2012. Alexander died a year ago at the age of 43.More news: Monitor says Syria seizes half of eastern Ghouta
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There is a new lead in the nerve agent poisoning of the former Russian General Staff Main Military Intelligence Directorate (GRU) officer, convicted for espionage in favor of Great Britain in Russia.
Other officers in white suits and gas masks were seen packing items from the cemetery into a yellow barrel.
DS Bailey was one of the first at the scene when the pair were found slumped in The Maltings on Sunday.
Analyst Mathieu Boulegue of the Chatham House think tank said it would be "extremely difficult" to pinpoint who committed the attack, and that the response could be a lot more complicated if it turned out that internal factions were responsible.
The police came to the conclusion that Skripal had been poisoned nervously-paralytic substance, whose name is not yet known.
"A well-equipped lab and a very experienced analytical chemist can do it, but it's not the sort of thing a chancer doing kitchen-sink chemistry can get away with", chemical weapons expert Richard Guthrie said.
The Times newspaper said British officials were talking to their United States and European counterparts about a co-ordinated retaliation, which could include "diplomatic, economic and military measures" if Moscow was behind the attack.
"We're accused not only of this, but we are accused of everything that goes wrong on this planet, according to our Western partners", Lavrov said.
The military have been drafted in as part of the investigation (Picture: Getty) Police have said that military assistance will continue as long as is necessary.
In response to questions over Russia's possible involvement, May has said that "if action needs to be taken then the government will do that".