Foreign Secretary Boris Johnson warned on Tuesday that Britain would respond “robustly” if it emerged that a government was behind the suspected poisoning of a former Russian double agent.

Sergei Skripal, a former colonel in Russian military intelligence who came to Britain in a spy swap in 2010, was found unconscious with his daughter Yulia in the southwestern English city of Salisbury on Sunday.

The pair, found on a bench outside a shopping centre, were treated for “suspected exposure to an unknown substance” and are currently in a critical condition in a local hospital.

Johnson told the House of Commons that it was too soon to establish the cause of the “disturbing” incident, which caused a major security alert in the normally quiet city.

But he noted “the echoes” with the 2006 poisoning in London of Kremlin critic Alexander Litvinenko, an attack that an inquiry ruled was likely ordered by President Vladimir Putin.

“I can reassure the House that should evidence emerge that implies state responsibility, then Her Majesty's Government will respond appropriately and robustly,” Johnson said.

He added: “Though I am not now pointing fingers, I say to governments around the world that no attempt to take innocent life on UK (United Kingdom) soil will go either unsanctioned or unpunished.”

Putin's spokesman Dmitry Peskov said earlier on Tuesday that it had no information on the “tragic situation”.

“We don't have information about what could be the cause, what this person did,” he said.

He said London had not made any requests for assistance in the investigation, but added: “Moscow is always ready for cooperation.”

'Hallmarks of Russian attack'

Police earlier revealed that a number of emergency services personnel required medical assessment after the incident, and one remains in hospital.

However, they stressed there was no immediate risk to public health.

A cordon remained in place on Tuesday where Skripal, 66, and his 33-year-old daughter were found, while a restaurant on a street nearby, Zizzi, was also closed in a “precaution”.

Specialists from the counter-terrorism police unit are assisting in the investigation.

Fears of the suspected poisoning revived memories of Litvinenko, an ex-Russian spy who was killed by radioactive polonium put in his tea in London.

A British enquiry ruled in 2016 that Putin “probably approved” the killing and identified two Russians Andrei Lugovoi and Dmitri Kovtun, as the prime suspects.

The incident caused a deep diplomatic split between London and Moscow, and after a thaw, tensions are rising again, fuelled by accusations of Russian cyber-attacks on the West.

The Chairman of the House of Commons Foreign Affairs Committee, Tom Tugendhat, warned the evidence pointed to Russia's involvement in the Salisbury incident.

“It is too early to say whether it is certain or not, but it certainly bears all the hallmarks of a Russian attack,” he said.

'Anti-Russian provocation'

Skripal was sentenced to 13 years in jail in Russia in 2006 for betraying Russian intelligence agents to Britain's MI6 secret service.

He was pardoned before being flown to Britain as part of a high-profile spy swap between Russia and the United States in 2010.

His collapse made the front pages of almost all Britain's newspapers on Tuesday, with the Daily Mail speculating that Skripal may have been the target of a revenge “hit” by former colleagues.

Litvinenko's widow, Marina, told The Times that watching footage of emergency responders in hazardous material suits “was kind of deja vu”.

William Browder, a British hedge fund manager who has campaigned against the Kremlin over the death in custody of his former employee Sergei Magnitsky, said his “first suspicion” was that Moscow was involved.

“This man was considered by the Kremlin to be a traitor to Russia,” he told AFP.

“They have a history of doing assassinations in Russia and abroad. And they have a history of using poisons, including in Britain.”

However, Lugovoi, who is an MP in the Russian parliament, responded to the British media reports by saying that Britain “suffers from phobias”.

“Because of the presidential elections [on March 18], our actions in Syria, the situation with Skripal could be spun into an anti-Russian provocation,” he told Interfax news agency.