Counter-terrorism officers are now involved in the investigation of an incident near the English city of Salisbury that has left two people in critical condition, London’s police force says.

“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 suspect two people found 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.

The pair are now in hospital.

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

“Based upon the number of casualties affected, it is not believed that there is a significant health risk to the wider public.”

The pair, both in their 40s, were initially believed to have taken heroin or crack cocaine from a contaminated batch of drugs. They are being treated at Salisbury District Hospital, which remains open as usual, police said.

The Skripals spent weeks in a critical condition in the same hospital before slowly recovering and being discharged.

More than three days since the two people were found, tests are still being conducted to ascertain what substance made them ill.

Amesbury lies 11 kilometres to the north of Salisbury, where the Skripals were found slumped unconscious on a bench on March 4.

Forensic police examine a grave site at Salisbury Crematorium in March. Photo: EPA

Police said sites in Amesbury and Salisbury that they believed the man and woman found in Amesbury had frequented would be cordoned off as a precaution.

A Public Health England (PHE) spokesman said there was not a significant risk to the wider public.

Britain blamed Russia for poisoning the Skripals with Novichok nerve agent, the first known offensive use of such a nerve agent on European soil since World War II.

Moscow denied any involvement and suggested Britain had carried out the attack to stoke anti-Russian hysteria.

The attack prompted the biggest Western expulsion of Russian diplomats since the Cold War as allies in Europe and the United States sided with Prime Minister Theresa May’s view that Moscow was either responsible or had lost control of the nerve agent.

-AAP