Police in the British town of Amesbury have declared a major incident over fears that two people found unconscious were exposed to an unknown substance.

Amesbury is in the county of Wiltshire and around 16 kilometers from Salisbury, where former Russian double agent Sergei Skripal and his daughter Yulia were poisoned earlier this year with what was later identified by the British Porton Down defense laboratory as Novichok. The United Kingdom blamed Russia for the act, but Russia has continuously denied any involvement.

The incident has dealt a heavy blow to diplomatic ties between the two countries.

Initially it was thought that the man and the woman, both in their 40s, were involved in a drugs-related incident. Police said they remain "open minded" about what the cause may be. UK counterterrorism police have started an investigation.

Although it is not yet clear if a crime has been committed, as a precaution, locations that they are known to have frequented both in Amesbury and Salisbury have been cordoned off.

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

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

It is thought that the couple attended a family fun day at a local church on Saturday afternoon before they were found unconscious. "There have been no reports of any other incidents," said Roy Collins, the church secretary. "We are all quite puzzled and shocked－naturally the connection with Salisbury and recent events there mean there is a heightened public interest."

He also said that no one else had reported any ill-effects after the event and as far as he was aware, "there was nothing going on that was nefarious".

Government ministers were "being kept up to date about the incident in Amesbury", said a spokesman, and Wiltshire's Police and Crime Commissioner Angus Macpherson said the police were continuing to "contain any risk that might be there".