National and local authorities said only that a man and a woman were found unconscious Sunday afternoon on a bench in a shopping mall in Salisbury, about 90 miles (145 kilometers) west of London.
British media identified him as Sergei Skripal, 66, who was convicted in Russia on charges of spying for Britain and sentenced in 2006 to 13 years in prison. Skripal was freed in 2010 as part of a U.S.-Russian spy swap.
Wiltshire Police, which is responsible for the Salisbury area, said the man and woman appeared to know one another and had no visible injuries. "They are currently being treated for suspected exposure to an unknown substance. Both are currently in a critical condition in intensive care," the police department said in a statement.