Video report by ITV News Correspondent Dan Rivers
Russian double agent Sergei Skripal and his daughter Yulia remain in critical condition after being exposed to a mystery substance that has prompted crisis talks in Whitehall.
Counter-terror officers extended the cordon further on Wednesday after the 66-year-old was found unconscious in Salisbury, Wiltshire, along with his 33-year-old daughter shortly after 4pm on Sunday.
Scotland Yard said authorities were "keeping an open mind as to what happened," and that the incident had not been declared a terrorist incident.
They have urged anybody who visited Salisbury town centre on Sunday afternoon or who visited a Zizzi restaurant or the Bishop's Mill pub to come forward.
The Government held a meeting of its emergency committee Cobra on Wednesday morning, chaired by Home Secretary Amber Rudd.
Following the cross-departmental meeting Ms Rudd said was known about the substance which is suspected to have poisoned Mr Skripal and his daughter Yulia.
The Home Secretary's statement came as relations between the UK and the Kremlin soured after Foreign Secretary Boris Johnson went on the offensive in the House of Commons.
Addressing MPs about the "disturbing incident", Mr Johnson said: "While it would be wrong to prejudge the investigation, I can reassure the House that should evidence emerge that implies state responsibility, then Her Majesty's Government will respond appropriately and robustly."
In a fresh sign of the deterioration in relations between the countries, Mr Johnson also claimed Russia is "in many respects a malign and disruptive force".
Britain will respond "appropriately and robustly" if Moscow is found to have had a hand in the incident, he said.
Russian Foreign Ministry spokeswoman Maria Zakharova said that "what happened to Mr Skripal has been immediately used to further incite anti-Russian campaign in Western media", adding that the allegations were intended to "further exacerbate relations between our countries.
The events surrounding Mr Skripal and his daughter have sparked unwelcome comparisons to the death of another former Russian spy, Alexander Litvinenko, who died in the UK in November 2006 after being poisoned, likely by Russian agents.
Angus MacPherson, the Police and Crime Commissioner for Wiltshire said: "It's particularly shocking that an incident such as this appears to have happened on a quiet Sunday afternoon in our cathedral city.
"Over the past couple of days I have been briefed regularly by the Chief Constable and can say with confidence the matter has received the full support of both the police and our partner agencies in a coordinated major response."
Metropolitan Police Assistant Commissioner Mark Rowley, head of Counter Terrorism Policing, said in a statement:" We would like to reassure members of the public that this incident is being taken extremely seriously and we currently do not believe there is any risk to the wider public."
Video report by ITV Political Correspondent Paul Brand
Temporary Chief Constable for Wiltshire Kier Pritchard confirmed the Metropolitan counter-terrorism network is now leading the investigation due to the incident's "unusual circumstances".
He added: "It is important to reiterate they have not declared this a terrorist incident as at this stage they are keeping an open mind as to what has happened.
"This has been a fast paced investigation and our focus has been on trying to establish what has caused these people to become critically ill and whether or not criminal activity has taken place."
What happened in Salisbury?
Mr Skripal, a former Russian spy, was found unconscious alongside his daughter Yulia near the Maltings shopping centre.
One member of the emergency services who was dealing with the incident is also in hospital.
"We can confirm that a small number of emergency services personnel were assessed immediately after the incident and all but one have been released from hospital," Wiltshire police said in an update.
Counter-terrorism specialists took over the investigation from Wiltshire police on Tuesday.
The Counter Terrorism Policing network has stepped in due to the "unusual circumstances" of the situation, Scotland Yard said, although the events have not been classified as a terrorist incident.
Police investigators are known to have recovered a CCTV image captured by a camera at the Snap Fitness 24/7 gym.
The image, from Sunday, shows a man and woman walking through an alleyway connecting the Zizzi restaurant and the bench where Mr Skripal and his daughter were found.
Freya Church, 27, who spotted the pair "slumped" on a bench, said the couple in the CCTV images were "100%" the same people she had seen.
"He was slumped ... and she was slumped into his chest, leaning on him. They looked like they had been there a little while," she told ITV News.
"To be honest I thought they were homeless people who had taken something a little bit too strong."
After Mr Skripal and his daughter were discovered, the police cordoned off a nearby Zizzi restaurant on Castle Street and the Bishop's Mill pub in the Maltings.
Police were still at the restaurant early on Tuesday morning, with an officer in an unmarked car outside, and both locations remained closed.
What has Russia said about the incident?
In Moscow, Kremlin spokesman Dmitry Peskov called Mr Skripal's illness a "tragic situation" but added "we don't have any information".
He said Moscow had not been approached to help in any investigation, but stood "ready to cooperate".
The Russian Embassy said in a statement: "The situation regarding the hospitalisation of Sergei Skripal and another person on March 4, as described by the British media, causes serious concern.
"As of today, the police or other British authorities have not provided the Embassy with any official information regarding this incident. The Foreign Office has not given any notifications either.
"However, the media are swiftly launching a new phase of the anti-Russian campaign ongoing in the UK. Readers are offered various speculations which ultimately lead to a vilification of Russia.
"Although UK law enforcement agencies have not given any substantive comments on this incident, media reports create an impression of a planned operation by the Russian special services, which is completely untrue.
"We believe that the British authorities and law enforcement bodies should step in immediately and inform the Embassy and the British society about the actual circumstances of this incident, so as to end the demonisation of Russia. With this in mind, the Embassy has turned to the Foreign Office for clarifications."
What are others saying about it?
Given the diplomatic tension raised by the killing of Mr Litvinenko in 2006, Labour MP Chris Bryant, who chairs the All-Party Parliamentary Group on Russia, said:
"We have got to be a little careful about establishing the facts ... but we know [Russian President Vladimir] Putin's record of using excessive violence," he said,
"There is a long list of Putin opponents who have been bumped off around the world. The fact that this happens just before [Russia's] presidential elections, I would suspect, is not circumstantial."
Yuri Felshtinsky, a former associate of Litvinenko, said the incident bore the hallmarks of a state-ordered assassination attempt.
"In the context of the Russian presidential election this has all hallmarks of a Putin assassination," he said.
Bill Browder, a Putin critic and anti-corruption campaigner, said the situation added up to a likely assassination attempt by the Russian security services.
"The only logical theory that we have right now is the theory that this was an assassination attempt by the Kremlin," he told ITV News.
"It may be disproven, but if you don't elevate it to the level where it's at right now then you end up not figuring out what's happening."
Conservative MP Bob Seely similarly told ITV News the authorities needed to "be wise" to the threat from Russia, and investigate the situation accordingly.
Richard Walton, the former head of Scotland Yard's counter-terrorism command, said that "if this is state-sponsored terrorism and it looks entirely possible, then it will have grave consequences for UK Russia bilateral relations".
Police, however, have not speculated on a link to the Kremlin. They are focused on identifying the substance that appears to have caused Mr Skripal's illness.
Who is Sergei Skripal?
Mr Skripal, 66, was imprisoned in Russia in 2006 for being a double agent. He was arrested in Moscow in December 2004 and jailed two years later for 13 years, but was granted refuge in the UK following a "spy swap" deal.
A retired Russian colonel he was recruited by British intelligence in the mid-1990s, reportedly receiving over $100,000 (£72,000) for his services.