A man and woman are in critical condition and are being tested for exposure to an unknown substance just miles from the British town where former Russian spy Sergei Skripal was poisoned in March.

Local police say they haven't found a connection to the Skripal case but have declared a "major incident" in the area out of precaution.

The man and woman, who have not been identified, were found unconscious on Saturday in Amesbury, seven miles from Salisbury where Skripal was exposed to a nerve agent.

And, initially it was believed they'd suffered from a reaction to heroin or cocaine. Police didn't raise the emergency level until late Tuesday night.

This church is one of several areas the two frequented that have been cordoned off.

It's known they often visited Salisbury, where the British government believes Russia's government tried to kill Skripal and his daughter.

Russia has steadfastly denied any connection.