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Boris Johnson vowed to respond "robustly" if evidence emerges of Russian involvement in the illness of former double-agent Sergei Skripal. The foreign secretary did not rule out imposing new sanctions against Moscow.

Skripal, 66, and his daughter, Yulia, 33, were found slumped on a shopping center bench in Salisbury, Wiltshire, on Sunday.

UK counter-terror police are currently racing to work out which, if any, "unknown substance" harmed the pair, who remain in a critical but stable condition. At least one of the emergency service workers remains in hospital after dealing with the incident.

Despite stressing that it was "wrong to prejudge the investigation," Johnson, who was several minutes late to the session, warned that if MPs' "suspicions (about Salisbury) prove to be well-founded, then it may very well be that we are forced to look again at our sanctions regime and other measures that we may seek to put in place."

"I am not now pointing fingers," Johnson added, "because we cannot point fingers, I say to governments around the world that no attempt to take innocent life on UK soil will go either unsanctioned or unpunished."

Moscow is ready to cooperate with the UK to investigate an incident which left Russian ex-double agent Sergey Skripal in critical condition. Skripal spied for UK and was part of a 2010 US-Russia "spy-swap."

Asked if London had been in touch seeking help with the investigation, Kremlin spokesman Dmitry Peskov said that "nobody has approached" Russia "with such a request." However, Moscow "is always open for cooperation," he told reporters on Tuesday morning.

Moscow doesn't have any details about what happened in Salisbury, Peskov said. "We don't have information about what this person was doing [in England]," he said, adding that Moscow doesn't know what citizenship Skripal currently holds.

Responding to British media speculation about Moscow being implicated in Skripal's poisoning, Peskov said: "I can't comment. But you know as they say, it wasn't slow to arrive."

On Sunday, Skripal was one of two people found to have been affected by "an unknown substance" in the city of Salisbury, England, according to local media. "The two people - a man aged 66, and a 33-year-old woman - were found unconscious on a bench," police said on Tuesday, adding that both are in critical condition. The pair "did not have any visible injuries," police added.

Later on Tuesday, UK Foreign Secretary Boris Johnson confirmed that the victims are Skripal and his daughter, Yulia. Though it's too early to speculate on the nature of the incident, there will be suspicions, according to the top politician. "If those suspicions prove to be well founded, then this government will take whatever measures we deem necessary to protect the lives of the people in this country, our values and our freedoms," he added.

In the meantime, the Russian embassy in the UK asked the British Foreign Office (FCO) to clarify the situation surrounding the Salisbury incident, an official from the embassy told TASS.

According to the local Salisbury Journal, the emergency services suspect fentanyl, a synthetic drug similar to heroin, only far stronger. It isn't clear how Skripal and his female companion were exposed to the drug. Fentanyl can endanger anyone who comes into contact with it, as it can be absorbed through the skin.

British media quickly drew parallels between the recent incident and the controversial case of Alexander Litvinenko, a former Russian security officer who died from radioactive poisoning in 2006. Former MI5 intelligence officer Annie Machon told RT that it's premature to judge. "The Russians would not have handed him over, this guy, back to the West if they still felt he could have caused damage. There seems to be little motivation to do anything against him," Machon said.

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According to him, "there will be no militant exit like in Aleppo in December 2016." "This is our land and we do not intend to leave," the spokesman added.