Counter-terrorism officers are now involved in the investigation of an incident near the southwestern English city of Salisbury which has left two people in critical condition, London's police force said on Wednesday.

"Given the recent events in Salisbury, officers from the counter terrorism network are working jointly with colleagues from Wiltshire Police regarding the incident in Amesbury," the Metropolitan Police said in a statement.

"As Wiltshire Police have stated, they are keeping an open mind as to the circumstances surrounding the incident."

Police in Wiltshire, where Russian ex-double agent Sergei Skripal and his daughter Yulia were poisoned with a nerve agent in March, took the rare step of declaring a major incident after a man and a woman were found unconscious on Saturday in Amesbury.

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