British media report an ex-Russian double agent was in a critical condition on Monday after being exposed to an "unknown substance."

Authorities did not identify the man, but multiple media outlets gave his name as Sergei Skripal, a 66-year-old former general of Russian military intelligence that had been convicted in Russia for spying for the UK.

Police said two people had been found unconscious on a bench on Sunday in downtown Salisbury, a city in the south of England.

"The pair, who we believe are known to each other, did not have any visible injuries and were taken to Salisbury District Hospital," the police said. They described the patients as a "man aged in his 60s and a woman aged in her 30s," adding they were both "in a critical condition."

A police presence was reported at Skripal's home on Monday.

Read more: US sanctions five Russians over human rights violations

A hospital in Salisbury said earlier on Monday that it was "dealing with a major incident involving a small number of casualties." It said staff and patients should come to the hospital as usual. However, the hospital's emergency room was closed.

Authorities deployed decontamination crews to the area where the pair was found, with workers in protective suits seen spraying down the street. The police also said a restaurant in downtown Salisbury "has been closed in connection with the on-going major incident."

The authorities also said there was no known risk to public health, but recommended people contact medical services "as a precaution" if they felt ill.

Former colonel in military intelligence

Skripal had reached the rank of a colonel in Russia's GRU military intelligence service before retiring from the post in 1999 to continue his career in Russia's foreign ministry. He was arrested in 2004 and later convicted and sentenced to 13 years for betraying agents to British intelligence. He was eventually swapped as part of a Cold War-style spy exchange in Vienna in 2010.

Relations between Britain and Russia have been strained since the murder of ex-KGB agent Alexander Litvinenko in London in 2006. A British inquiry said that the "hit" was probably approved by President Vladimir Putin.

Meanwhile, police said they were "conducting extensive inquiries to determine exactly what led to these people falling unconscious."

"This has not been declared as a counter-terrorism incident and we would urge people not to speculate," Wiltshire police's Temporary Assistant Chief Constable Craig Holden told reporters.

"However, I must emphasize that we retain an open mind, and that we continue to review this position."

dj,av/se (AFP, Reuters)

Each evening at 1830 UTC, DW's editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.