UK police declare 'major incident' as 2 people fall ill near where former Russian spy was poisoned

British authorities declared a “major incident” late Tuesday after it was determined that two people in critical condition may have been exposed to an unknown substance near the southern English town of Salisbury.

The town is notable because it was the location where a former Russian spy and his daughter were poisoned with nerve agent earlier this year.

Wiltshire police said the pair, a man and woman in their 40s, remains in critical condition after being found in Amesbury.

“Emergency services were called to an address in Muggleton Road, Amesbury on Saturday evening (June 30) after a man and woman, both in their 40s, were found unconscious in a property,” the police said.

The town is just miles away from Salisbury, where former Russian spy Sergei Skripal and his daughter Yulia were poisoned with nerve agent in March. The Skripals eventually recovered from the poisoning.

The British government at the time accused the Russian government of facilitating the attack on the British soil, sparking a major diplomatic standoff and expulsion of dozens of diplomats from both countries. Russia denied any involvement in the poisoning.

Former Russian spy Sergei Skripal and his daughter Yulia Skripal.

Former Russian spy Sergei Skripal and his daughter Yulia Skripal. (AP)

Police on Wednesday were cordoning off places the exposed pair visited before becoming ill and they will remain closed overnight as a precaution.

The authorities initially suspected the two people got ill after using "possibly heroin or crack cocaine from a contaminated batch of drugs,” AFP reported.

"However, further testing is now ongoing to establish the substance which led to these patients becoming ill and we are keeping an open mind as to the circumstances surrounding this incident," it added.

Public health officials said it’s unlikely the wider public is in danger.

“The current advice from PHE England, based upon the number of casualties affected, is that it is not believed that there is a significant health risk to the wider public. This will be continually assessed as further information becomes known,” a Public Health England (PHE) spokesman told Reuters.

The man and woman in critical condition are currently being treated at Salisbury District Hospital, which will remain open as usual, according to the police. It is the same hospital the Skripals were treated at.

The Associated Press contributed to this report.