British police say unknown substance leaves two people critically ill near Salisbury

British police declared a major incident after finding a man and a woman in a critical condition who may have been exposed to an unknown substance near the southern English town of Salisbury.

Police in Wiltshire, where former Russian spy Sergei Skripal and his daughter Yulia were poisoned with a nerve agent in March, said a man and a woman were found unconscious on Saturday in Amesbury.

"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," police said late on Tuesday.

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.

"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," police said.

"At this stage it is not yet clear if a crime has been committed. A police investigation has been established. In addition, a full multi-agency response has been co-ordinated."

Amesbury lies seven miles (11 kms) to the north of Salisbury, where the Skripals were found slumped unconscious on a bench on March 4.

Police said sites in both 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.