Specialist officers from the counter-terrorism squad are helping investigate the incident, which also left a 33-year-old woman -- reported to be Skripal's daughter Yulia -- in a critical condition in what is feared to be a poison plot.

Skripal, a 66-year-old former colonel in Russian military intelligence, is also in a critical condition in Salisbury District Hospital.

Police did not confirm their names but did confirm that two people aged 66 and 33 were being treated for "suspected exposure to an unknown substance".

They also revealed that a member of the emergency services who helped deal with the incident remained in hospital.

A "major incident" was declared and the area around the bench where the couple was found slumped remained cordoned off on Tuesday, while a restaurant on a street nearby, Zizzi, was also closed in a "precaution".

The case revived memories of the death of Alexander Litvinenko, an ex-Russian spy and Kremlin critic was who poisoned in 2006 with radioactive polonium in London on orders from Moscow.

The incident caused a deep diplomatic split between London and Moscow, and another death blamed on Russia would ratchet up tensions even further.

The chairman of the UK House of Commons foreign affairs committee warned that was where the evidence was pointing.

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

- 'Very unusual case' -

Britain's chief counter-terrorism officer, Mark Rowley of London's Metropolitan Police, said specialist members of his team were supporting the investigation.

"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," he told BBC radio.

Local police say they are keeping "an open mind", adding that they did not know yet if a crime had been committed, but said there was no risk to the public.

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.

A British inquiry ruled in 2016 that Russian President Vladimir Putin "probably approved" the killing and identified two Russians, Andrei Lugovoi and Dmitri Kovtun, as the prime suspects.

Putin's spokesman Dmitry Peskov said Tuesday that it had no information on the Salisbury incident.

"We see that such a tragic situation happened," he said, adding: "But we don't have information about what could be the cause, what this person did."

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

- 'Traitor to Russia' -

The incident 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 newspaper 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.

Rowley noted 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," he said.

AFP