Police are continuing to investigate the "unknown substance" that left a former Russian double agent and his daughter in a critical condition in hospital.
Sergei Skripal, 66, and his 33-year-old daughter, Yulia, were found unconscious on a bench in Salisbury on Sunday.
Several members of emergency services personnel who responded to the call were themselves taken ill, with one remaining in hospital.
Investigators have seized CCTV footage from a nearby gym showing a man and woman, believed to be the victims, walking nearby just half an hour before police were called.
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The Russian Embassy posted a short response to Mr Johnson's address to MPs, saying ironically: "We are impressed by the statement of the Foreign Secretary in Parliament today. The Foreign Secretary spoke in such a manner as if the investigation was already over and Russia was found responsible for what had happened in Salisbury."
The statement continued: "We regret that instead of a proper official clarification on the issue the Foreign Secretary chose to threaten Russia with retribution. Looks like the script of yet another anti-Russian campaign has been already written."
An unnamed ex-radiation biologist said in a comment issued through the Science Media Centre: "If the outcome is not just something like a couple of dodgy kebabs – again less probable with both individuals being symptomatic at the same time and with considerable rapidity if they had just eaten there – then I would look for a chemical source.
"Decontamination at the scene would also suggest that possibility. However, we shouldn't totally ignore biological contamination of food or the environment.
"However, the latter would have caused a wider response from PHE and the authorities."
A military research facility is thought to be conducting tests after Russian double agent Sergei Skripal and his daughter Yulia were exposed to an unknown substance.
The Defence Science and Technology Laboratory in Porton Down, which has state-of-the-art equipment to look for trace amounts of substances, is believed to be involved in examining what could have caused them to fall ill.
Salisbury MP and city minister John Glen told the BBC: "Fortunately just down the road in my constituency at Porton Down defence, science and technology labs exists and they will have taken the substance and will be trying to evaluate what they can, no doubt."
The Russian Embassy in London has said the country is being "demonised" in news coverage of the incident and has contacted the Foreign Office to complain.
A spokesperson said: “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.”
The Prime Minister and senior ministers have been updated on the investigation at a meeting of the National Security Council in the Cabinet room. It followed a meeting of senior officials earlier in the day, Downing Street said.
The long history of Russian deaths in the UK under mysterious circumstances
The case of a former Russian colonel and former spy for Britain who remains in critical condition after being exposed to an unknown substance, has drawn comparisons with other suspicious events involving former Kremlin operatives in the UK.
So, still nothing on toxicology tests or any changes in the condition of Mr Skripal and his daughter.
There has been speculation regarding what "unknown substance" may have caused their illness, none of which has confirmed.
Some experts have suggested the use of nerve agents because of a witness report describing Mr Skripal and his daughter vomiting, twitching and unable to move.
Symptoms such poisoning include convulsions and deaths by asphyxiation because control of respiratory muscles is lost as a result of the attack on the nervous system.
Among the most powerful is VX, a toxin developed at the British Porton Down facility, which was used to assassinate Kim Jong-nam – the brother of North Korean leader Kim Jong-un, at Kuala Lumpur International Airport in February 2017. The suspect accused of handling the toxic was taken ill with vomiting because of the exposure.
Other nerve agents include sarin, which has similar effects and can lead to death by asphyxia. The chemical weapon has been used in gas form against civilians by regime forces in the ongoing Syrian civil war.
A press conference is now starting at Wiltshire Police's conference in Salisbury now, where they will give updates on the "co-ordinated major response".
The Temporary Chief Constable, Keir Pritchard, said local authorities, the NHS, police and a "wide range of partners" are working together.
"Due to the unusual circumstances the Counter Terrorism Network will be leading this investigation as it has the specialist capabilities to do so," he says.
"They have not declared this a terrorist incident."
T/CC Pritchard said there is no wider risk to the wider public but confirms that a small number of police officers and emergency services personnel needed medical assistance.
Photos have emerged of Sergei Skripal's 33-year-old daughter, Yulia.
They both remain critically ill in hospital.