Two people critically ill after exposure near Salisbury where ex-spy Skripal and daughter were poisoned by nerve agent.

UK counterterrorism police have begun investigating a possible poisoning that left two people in critical condition after they were exposed to an unknown substance near the southern town of Salisbury.

“Wiltshire Police and partners have declared this evening a major incident after it is suspected that two people might have been exposed to an unknown substance in Amesbury,” the police said in a statement on Tuesday.

The police said emergency services were called to Amesbury on Saturday evening after a man and woman, both in their 40s, were found unconscious in a property.

Citing medical sources, The Sun newspaper reported on Wednesday that the duo was showing “very similar” symptoms to those suffered by Russian former double agent Sergei Skripal and his daughter Yulia.

“Doctors have determined that the pair have been poisoned by toxins and are showing very similar … symptoms to the former Russian military intelligence colonel and his daughter,” the newspaper said.

There was no word on the identity of the couple.

Amesbury lies 11km to the north of Salisbury, where in March, Skripal and his daughter, were found unconscious on a bench near a shopping mall.

British officials say they were poisoned by a nerve agent.

Police on Wednesday said sites that it believed the man and woman found in Amesbury had frequented in both Amesbury and Salisbury would be cordoned off overnight as a precaution.

It was initially thought the man and woman had possibly used crack cocaine or heroin from a batch of contaminated drugs, police said, adding that further testing was ongoing.

“The current advice from Public Health England (PHE), 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 PHE spokesperson said.

The man and woman were being treated at Salisbury District Hospital, which remains open as usual, police said.

Britain accuses Russia of poisoning the Skripals with Novichok nerve agent; Moscow denies the allegation.