Media reports in Britain say a former Russian spy is among two people who are in a critical condition after they were exposed to an unknown substance yesterday.
It is reported that one of the people being treated in hospital is Sergei Skripal, a former Russian spy given refuge by Britain in 2010.
The 66-year-old was convicted in Russia of passing state secrets to Britain in 2006 and sentenced to 13 years in prison.
The former colonel in Russian military intelligence was one of four people exchanged in 2010 for ten Russian agents deported from the US in an historic spy swap.
Wiltshire Police said they are investigating the circumstances after an incident in Salisbury, but have reassured people there is no risk to the wider public.
Mr Skripal and a woman in her 30s are being treated in hospital after police found them unconscious on a bench in The Maltings shortly after 4pm yesterday.
Police said: "At this stage it is not yet clear if a crime has been committed and a multi-agency response has been coordinated.
"Police are carrying out a full investigation and working with partner agencies to clarify the exact circumstances.
"At this stage, Wiltshire Police does not believe there is any risk to the wider public."
The force said a number of areas in Salisbury had been cordoned off in relation to their investigation.
A spokesman for Public Health England said anyone exposed to the unknown substance had been decontaminated "as is standard practice in situations like this".
Relations between Britain and Russia have been strained since the murder of ex-KGB agent Alexander Litvinenko in London in 2006, a killing which a British inquiry said was probably approved by Russian President Vladimir Putin.
