The road is blocked near a police tent in The Maltings shopping centre in Salisbury, England, near where Sergei Skripal was found critically ill by exposure to an unknown substance. Picture: Steve Parsons/PA via AP

Moscow - The Kremlin offered on Tuesday to help British authorities investigate the mysterious illness of a former Russian double agent believed to have been exposed to an unknown toxic substance.

"Moscow is always open for cooperation," Kremlin spokesman Dmitry Peskov said in comments carried by state news agency TASS.

However, "we do not have information about what the reason could be or what this man was engaged in that could have been connected with it [the incident]," Peskov said.

British media reports said the former Russian intelligence officer, Sergei Skripal, 66, and a female companion in her 30s were hospitalized in critical condition after being found unconscious on a bench in the south-west English city of Salisbury on Sunday.

Skripal, a former colonel in Russian military intelligence, was imprisoned in Russia in 2006 on allegations of spying for Britain.

He was released four years later to live in Britain as part of a high-profile spy-swap deal with the United States, under which the US released 10 agents planted by Russia.

The mysterious illness has drawn comparisons to that of another Russian former intelligence officer, Alexander Litvinenko, who died in 2006 after drinking tea laced with a radioactive isotope.

A British inquiry concluded that Litvinenko was targeted for assassination by the Russian intelligence services. Litvinenko's death soured relations between London and Moscow for many years.