The Kremlin said on Tuesday that it had no information on a former Russian double agent who fell ill in Britain after exposure to an unknown substance, calling the incident "tragic".

"We see that such a tragic situation happened," President Vladimir Putin's spokesman Dmitry Peskov told journalists. "But we don't have information about what could be the cause, what this person did."

File photo: Kremlin spokesman Dmitry Peskov during a meeting in Moscow, Russia June 21, 2017. (Photo: Reuters)

Peskov added that London has not made any requests for assistance in the probe which police had launched after the 66-year-old former Russian military intelligence colonel Sergei Skripal was found unconscious on a bench in Salisbury, about 145 kilometers west of London.

Asked if Moscow was ready to cooperate, Peskov said that "Moscow is always ready for cooperation".

He said he did not know whether Skripal was still a Russian citizen.

Public records list Skripal as having an address in Salisbury.

Skripal served with Russia's military intelligence, often known by its Russian-language acronym GRU, and retired in 1999. He then worked at the Russian Foreign Ministry until 2003 and later became involved in business.

After his 2004 arrest in Moscow, he confessed to having been recruited by British intelligence in 1995 and said he provided information about GRU agents in Europe, receiving more than $100,000 in return.

Skripal was sentenced to 13 years in prison for espionage in 2006 before being granted refuge in Britain in a high-profile US-Russian spy swap in 2010.

The case has evoked parallels with poisoning of an ex-Russian spy Alexander Litvinenko in London with radioactive polonium.

Commenting on the media reports, Andrei Lugovoi, an MP in the Russian parliament, said that Britain "suffers from phobias" and could use the incident to harm Russia ahead of the March 18 presidential polls.

"Because of the presidential elections, our actions in Syria, the situation with Skripal could be spun into an anti-Russian provocation," he told Interfax news agency.