A man reportedly convicted of spying against Russia is critically ill after being exposed to an unknown substance in Britain, as police declared a major incident.

Former Russian colonel Sergei Skripal, 66, who was granted refuge in the UK following a “spy swap” between the United States and Russia in 2010, has been named by the BBC as one of two people rushed to hospital after behaving oddly in a Salisbury shopping centre.

“Police received a call at approximately 4.15pm yesterday regarding concern for the welfare of a man and a woman in The Maltings [shopping centre], Salisbury,” police said in a statement Tuesday morning (AEDT).

“They were taken to Salisbury District Hospital and are being treated for suspected exposure to an unknown substance. They are currently in a critical condition.”

A woman, thought to be in her 30s, was also treated for contamination with Mr Skripal. The substance is still unknown.

A number of locations in the city centre were cordoned off and the emergency department of Salisbury District Hospital was closed.

The hospital tweeted: “Salisbury NHS is currently dealing with a major incident involving a small number of casualties, with a multi-agency response.

“We are not asking additional staff to come to site unless contacted directly.”

Mr Skripal was jailed for 13 years in 2006 for spying for Britain, the BBC reported.

He was convicted of passing the identities of Russian intelligence agents working undercover in Europe to the UK’s Secret Intelligence Service, MI6.

Russia at the time said Mr Skripal had been paid US$100,000 ($130,000) for the information, which he had been supplying since the 1990s.

He was one of four prisoners released by Moscow in 2010 in exchange for 10 US spies as part of a swap, the BBC said.

A Public Health England spokesman earlier said onlookers exposed to the substances had been decontaminated.

“Scientists from PHE’s Centre for Radiation, Chemical and Environmental Hazards will continue to assist the response and review information as it becomes available.”

Dorset and Wiltshire Fire Service were called to the hospital’s emergency department to help with the decontamination.