Amber Rudd has said more is now known about the substance which is suspected to have poisoned a Russian MI6 spy and his daughter, hinting that further details would be released later today. 

The Home Secretary made the announcement after an emergency Cobra meeting at Whitehall on Wednesday, and as Sergei Skripal, 66, remains critically ill in intensive care along with his 33-year-old daughter, Yulia.

Rudd said: “There will be more information published by the police this afternoon about the substance.”

“I want to make sure that this investigation responds to evidence, not to rumour, but I can reassure the public … that all action is going to be taken to keep everybody safe,” she added. 

Detectives investigating the incident on Wednesday urged witnesses and anyone with information to contact police “immediately” on 999 as they focus on establishing “what has caused these people to become critically ill”.

Skripal and Yulia were found unconscious outside a Maltings shopping centre in Salisbury on Sunday afternoon, after exposure to an unknown substance, now being investigated by counter-terrorism police.

The Defence Science and Technology Laboratory in nearby Porton Down, which has state-of-the-art equipment to look for trace amounts of substances, is examining what could have caused Skripal and his daughter to fall ill.

BBC Diplomatic Editor Mark Urban told Newsnight on Tuesday that authorities still have no idea what substance was used, saying, “they are very worried”.

“One [official] said to me – ‘we are treating symptoms, rather than causes, and that is not a good direction to be going in.’ Another person said to me that Col Skripal is ‘not at all in a good way’ this evening. There is a lot of concern I think that their condition could worsen.”

The Times said it was understood that scientists have ruled out radioactive substances and that there was speculation that a chemical such as an opiate could have been used. 

Toxicologists are said to be examining samples of blood, urine and tissue taken from the victims.

One former radiation biologist told MailOnline that the “considerable rapidity” of their decline suggested a chemical source. “Decontamination at the scene would also suggest that possibility,” he added.

“However, we shouldn’t totally ignore biological contamination of food or the environment. However, the latter would have caused a wider response from Public Health England and the authorities.”

A witness who saw Skripal and his daughter slumped on the park bench, told Sky News they did not appear they were in any pain. “They were just slumped. They were comfortable, they weren’t in any pain or anything. Lifeless, if you like.

“When you walked past, they didn’t even acknowledge that you’ve walked past. They just stayed slumped.”

Scotland Yard said detectives were “keeping an open mind as to what happened”, and that the incident had not been declared a terrorist incident – adding that there was no risk to the wider public, despite cordons being extended. 

Officers have carried out CCTV enquiries and spoken to a number of people as part of the inquiry, and they are now appealing to anybody who visited Salisbury town centre and surrounding areas on Sunday afternoon and has not yet spoken to police to get in touch.

They are keen to speak to anyone who visited Zizzi restaurant on Castle Street and The Bishop’s Mill pub in The Maltings.

Assistant Commissioner Mark Rowley, the head of Counter Terrorism Policing, said: “The focus at this time is to establish what has caused these people to become critically ill. 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.”

Meanwhile, Yvette Cooper, chair of the Home Affairs Select Committee, has written to Home Secretary Amber Rudd calling for an investigation into 14 deaths “that have not been treated as suspicious by the UK police, but have – reportedly – been identified by United States intelligence sources as potentially connected to the Russian state”.

The Russian Foreign Ministry on Wednesday said allegations of the country’s involvement in the incident were “designed to complicate UK-Russian relations”. 

The Cobra meeting will be chaired by Rudd and comes as as relations between the UK and the Kremlin soured after Foreign Secretary Boris Johnson went on the offensive in the House of Commons on Tuesday. 

It has since been suggested that if Russian involvement is proved, dignitaries including the Duke of Cambridge, could boycott the World Cup in Russia.

Addressing MPs about the “disturbing incident”, Johnson noted that this case had “echoes” of the death of Alexander Litvinenko, a Russian dissident who was fatally poisoned in London in 2006. 

He 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, Johnson also claimed Russia is “in many respects a malign and disruptive force”.

The Russian Embassy said it was “completely untrue” to suggest the country’s special services were involved and criticised Johnson for speaking “in such a manner as if the investigation was already over”.

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 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.

When Skripal and his daughter were found they did not have any visible injuries.

They were taken to Salisbury District Hospital, where they are being treated in intensive care for “suspected exposure to an unknown substance”.

Officers subsequently “secured” a number of scenes – including the Zizzi restaurant on Castle Street and the Bishop’s Mill pub in The Maltings.

At least two people left a contamination tent inside the cordon wearing protective suits and gas masks on Tuesday night.

People could also be seen inside Zizzi’s restaurant wearing protective gear and masks.