A major incident has been declared in the English county of Wiltshire after it was suspected that two people left fighting for their lives might have been exposed to an "unknown substance".

The man and woman, both in their 40s, were found unconscious at a property in Amesbury - close to Salisbury, where former Russian spy Sergei Skripal and daughter Yulia were poisoned by a suspected military nerve agent in March.

Wiltshire Police initially thought the pair's collapse on Saturday was drugs-related, but have since said it is suspected they "might have been exposed to an unknown substance".

Detectives are "open-minded" as to the cause and further tests are being carried out.

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The man and woman are in a critical condition at Salisbury District Hospital, which is where the Skripals were treated after their poisoning earlier this year.

Several places in the Amesbury and Salisbury area - including Amesbury Baptist Church - that are believed to be areas the pair visited before they fell ill were cordoned off on Wednesday while authorities investigate.

Public Health England (PHE) said it did not believe there to be a "significant health risk" to the wider public, although its advice was being continually assessed.

Police said it was not yet clear if the pair were the victims of a crime, but detectives are keeping an "open mind" around the circumstances.

Sergei Skripal with his daughter Yulia
Sergei Skripal with his daughter Yulia

Amesbury is about eight miles from Salisbury, where the Skirpals were poisoned. The British government has accused Russia of being behind the attack on Skripal, who settled in the UK after a spy swap, and his daughter.

The pair in Amesbury were found unconscious at an address in Muggleton Road on Saturday evening and it was at first believed they might have taken Class A drugs.

"It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs," police said.

"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."

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A PHE spokesman said: "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".

Areas pair visited before they fell ill cordoned off

Muggleton Road is on a new housing development on the southern edge of Amesbury, which lies close to Stonehenge.

A number of scenes believed to be areas the pair visited before they fell ill were cordoned off overnight.

Meanwhile, there was an increased police presence in and around Amesbury and Salisbury.

Salisbury District Hospital was "open as usual" and officials advised people to attend routine appointments unless they are contacted to do otherwise.

Police statement as 'major incident' declared

A police statement said: "Wiltshire Police and partners have this evening declared a major incident after it is suspected that two people might have been exposed to an unknown substance 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.

"They are both currently receiving treatment for suspected exposure to an unknown substance at Salisbury District Hospital. They are both in a critical condition.

"It was initially believed that the two patients fell ill after using possibly heroin or crack cocaine from a contaminated batch of drugs.

"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."

Salisbury locked behind cordons for two months after Skripals poisoning

The Amesbury incident comes almost two months after police finished investigating every area linked to the Skripal poisoning case in Salisbury.

Parts of the Wiltshire city were locked behind police cordons for more than two months as officers and soldiers worked to gather evidence and clean up areas affected by the Novichok nerve agent.