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."
BBC reported that two Wiltshire police officers dealing with the incident had been admitted to hospital yesterday with "minor symptoms".
The BBC said they had itchy eyes and wheezing and were released on Monday afternoon.
Wiltshire Police said: "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."
The statement said that a number of places - including the Zizzi restaurant and the Bishop's Mill pub in The Maltings - have been "secured" as part of the investigation
"At this time, we cannot confirm how long these cordons will remain in place."
The statement added: "We have access to a wide range of specialist resources and services that are helping us to understand what we are or aren't dealing with at this time.
"We would continue to appeal to any members of the public who may have information in relation to this incident to contact us immediately via 101 or 999."
A former Russian double agent is critically ill in hospital following suspected exposure to an unknown substance, as counter-terrorism police probe what caused him to collapse.
Sergei Skripal, 66, was found unconscious in Salisbury, Wiltshire, along with a woman in her 30s, reported by the BBC to be his daughter, Yulia, shortly after 4pm on Sunday.
As CCTV believed to show the pair in the moments before they were found slumped on a bench emerged, the UK's top counter-terrorism officer, Metropolitan Police Assistant Commissioner Mark Rowley, said: "We have to be alive to the fact of state threats."
Russia said it had no information regarding the "tragic situation" but is willing to co-operate, as comparisons were drawn with the death of dissident Alexander Litvinenko in 2006.
Andrei Lugovoi, the man suspected of poisoning Mr Litvinenko, dismissed the reports as propaganda.
Mr Skripal was convicted in 2006 of passing state secrets to MI6 before being given refuge in the UK as part of a spy swap.
The former colonel in the Russian military intelligence, who was sentenced to 13 years in prison, was among four convicts who were given pardons and one of two sent to Britain in 2010 in a deal that was said at the time to be the largest exchange since the Cold War.
The Castle Street branch of pizza chain Zizzi was closed late on Monday night "as a precaution" as part of the police investigation.
Wiltshire Police said officers were as yet "unable to ascertain" whether the pair, who are both in a critical condition in intensive care, have been victims of a crime.
Temporary Assistant Chief Constable Craig Holden, of Wiltshire Police, said: "The pair, who we believe are known to each other, did not have any visible injuries and were taken to Salisbury District Hospital.
"This has not been declared as a counter-terrorism incident and we would urge people not to speculate. However, I must emphasise that we retain an open mind and we will continue to review this position."
Igor Sutyagin, who was part of the same swap deal as Mr Skripal and is now a research fellow at RUSI, also urged caution.
He told the Associated Press: "There are lots of former security officers that deserted to the West. It is necessary to balance this information."
Salisbury District Hospital declared a major incident but told patients to attend appointments as normal unless advised otherwise.
A spokesman for Public Health England (PHE) said anyone exposed to the unknown substance had been decontaminated "as is standard practice in situations like this".
A CCTV image of a man and woman walking through an alleyway connecting the Zizzi restaurant and the bench where Skripal was found is believed to be of interest to police.
Police took away an image, shot at 3:47pm on Sunday, of a pair in footage, from a camera at Snap Fitness 24/7, according to the gym's manager, who showed the footage to the Press Association.
Cain Prince, 28, said: "Police had a good look at the footage and were interested in these two people. It was the only image they took away.
"They wanted a list of everyone in the gym between 3pm and 4pm as well."
Mr Prince added police said Skripal was "wearing a green coat".
British shadow home secretary Diane Abbott told BBC Radio 4's Today: "It's important not to speculate without knowing everything about it, but it does bear a striking similarity to the death of Litvinenko, who was poisoned by the Russian state, and before that Markov who was killed bizarrely by somebody stabbing him with an umbrella with poison on the tip."
The Labour frontbencher said she would write to Home Secretary Amber Rudd to ask what assurances she can provide "if it does prove to be the case that the Russian state is involved".
"I don't like defaulting to a 'red menace' analysis, but we can't allow London and the Home Counties to become a kind of killing field for the Russian state and its enemies," she said.
Freya Church, 27, who spotted the pair "slumped" and "passed out" on the bench told the Press Association the couple in the CCTV images were "100%" the people she saw on Sunday.
The gym worker, from Salisbury, said: "She was leaning on him, slumped.
"She looked passed out and he was looking up doing these hand movements (gesticulating upwards with arms).
"His eyes were glazed. To be honest I thought they were just homeless."
Police are still at a Salisbury restaurant closed in the wake of the contamination scare linked to former Russian double agent Sergei Skripal.
An officer sat in an unmarked car outside Zizzi in Castle Street , which was "secured as a precaution" on Monday night.
Mr Skripal, 66, and a woman in her 30s were found slumped on a bench near a children's play area about 100 yards away, close to The Maltings shopping centre.
They are both critically ill after being exposed to an unknown substance.
The UK's top counter-terrorism officer said his specialists were supporting the investigation.
Metropolitan Police assistant commissioner Mark Rowley said: "Clearly it's a very unusual case and the critical thing is to get to the bottom of what has caused this incident as quickly as possible.
"As you would expect, the specialist resources that sit within the counter-terrorism network that I coordinate across the country and other partners are working with Wiltshire Police to get to the bottom of that as quickly as possible."
He told BBC Radio 4's Today programme: "If you look back at other cases like (Alexander) Litvinenko, if necessary we will bring that investigation into the counter-terrorism network.
"At the moment the key is, though, to get to the bottom of what caused this."
Mr Rowley told Today: "We are doing all the things you would expect us to do. We are speaking to witnesses, we are taking forensic samples at the scene, we are doing toxicology work.
"That will help us get to an answer, I can't say any more at this stage."
Asked about a series of suspicious Russian-linked deaths in the UK, Mr Rowley said: "There are deaths which attract attention.
"I think we have to remember that Russian exiles are not immortal, they do all die and there can be a tendency for some conspiracy theories.
"But likewise we have to be alive to the fact of state threats as illustrated by the Litvinenko case."
Kremlin spokesman Dmitry Peskov called Mr Skripal's illness a "tragic situation" but added "we don't have any information".
He said no-one had approached them for help in the investigation but said "Moscow is always open to cooperation".
Asked about the link being made in the media between Mr Skripal and the death of Mr Litvinenko, Mr Peskov said: "It didn't take them long."