British police declared a major incident late on Tuesday after it said a man and a woman in a critical condition may have been exposed to an unknown substance near the southern English town of Salisbury.
"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," the police said in a statement.
"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.
Amesbury lies seven miles (11 kms) to the north of Salisbury, where in March Russian former spy Sergei Skripal and his daughter Yulia, fell severely ill after being poisoned by 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.
A Public Health England (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."
The man and woman were being treated at Salisbury District Hospital, which remains open as usual, police said.
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