London: British police said on Wednesday they suspected two people found in a critical condition near the English city where a former Russian spy was poisoned earlier this year may have been exposed to an unknown substance.

Police in Wiltshire, where 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.

Police officers stand outside a Zizzi restaurant in Salisbury in March. Credit:AP

The pair are now in hospital in a critical condition.

"They are both currently receiving treatment for suspected exposure to an unknown substance," Wiltshire Police said in a statement.