Two people are critically ill in hospital after suspected exposure to an "unknown substance" near the English town where a former Russian spy was poisoned with a nerve agent, officials said yesterday.

Counter-terrorism police, who the led the investigation into the poisoning of Sergei Skripal and his daughter Yulia, said they were assisting local police in the investigation which has been declared a "major incident" by the emergency services.

Scientists at Britain's defence laboratory at Porton Down are carrying out tests to try and establish if there is any connection between the two incidents, British media reported.

The couple, a man and a woman in their 40s, were discovered unconscious at a house in a quiet, newly-built area in Amesbury, a village near the prehistoric monument of Stonehenge on Saturday.

Local man Sam Hobson, 29, told AFP he was a friend of the pair, identifying them as Charlie Rowley and Dawn Sturgess.

Amesbury is about 12 kilometres (eight miles) from Salisbury, where the Skripals were found slumped on a bench in March in an incident that sparked a bitter diplomatic crisis with Russia.