Counterterrorism police requested the deployment of military personnel to help "remove a number of vehicles and objects" in Salisbury, London's Metropolitan Police said.
Sergei Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the attack on Sunday in the sleepy south-western English city of Salisbury.
A police officer who needed intensive care after attending the scene of a poisoning in Salisbury has said he does not consider himself a "hero" and he was "merely doing his job".
Downing Street said investigators had been summoned to provide updates on the case at 3pm on Saturday.
Britain's Home Secretary Amber Rudd, who visited Salisbury on Friday, said they were both still in a very serious condition, five days after collapsing.
Sky News Crime Correspondent Martin Brunt said police believe all three victims may have been contaminated at Mr Skripal's house in Salisbury and "ingested the poison some hours before they were taken ill".
Sergei Skripal, 66, and his 33-year-old daughter Yulia smiled for the camera while holding glasses of lager and wine inside the Italian chain.
"This was attempted murder in the most cruel and public way".
Wiltshire Police undated handout photo of Detective Sergeant Nick Bailey who rushed to the aid of a Russian ex-spy targeted with a nerve agent.
Investigators are said to be probing whether Yulia could have brought the toxin with her from Russian Federation as a present from friends for her father.
The grave of Skripal's wife, Liudmila, who died in 2012 from cancer, was also sealed off.
The executive director of the Europe-wide police agency Europol, Rob Wainwright, described the use of a nerve agent to attack a former Russian spy in the United Kingdom as an "outrageous affront to our security in Europe and our way of life", but added that people should "exercise caution before jumping to any conclusions".
Authorities are racing to identify the nerve agent used against the 66-year-old Skripal, who came to Britain in a spy swap in 2010, as politicians warned it showed the hallmarks of an attack by Russian Federation.
Noting it was "highly likely" that the officer, Bailey, was exposed to the same nerve agent as the Russian pair, she said Britain "will act without hesitation as the facts become clearer".
Moscow has reacted angrily to the accusations it was involved, with Foreign Minister Sergei Lavrov on Friday dismissing them as baseless "propaganda".
Scotland Yard said it was not exhuming a body, but would not comment further on the investigation, saying only the tent was in place for "operational reasons".
Police said 21 people had been seen for medical treatment since the incident.
A British public inquiry found that Putin had probably ordered Litvinenko's murder.
Russian officials deny the Kremlin had anything to do with the assassination attempt.
On Friday, John Glen, a member of Parliament for Salisbury, praised the calm response of the city's residents to "an unprecedented occurrence in our country's history" but acknowledged that some were demanding "decisive action".