LONDON - A man and woman feared to have been exposed to an unknown substance are in a critical condition in Amesbury, which is near former Russian spy poisoning site, prompting British police to declare a major incident on Wednesday.

The pair, in their 40s, were found unconscious in what was thought to be in a drugs-related incident in Wiltshire on Saturday, local press reported.

It is understood tests are being carried out on the substance at a government chemical weapons laboratory.

Amesbury is about 16 kilometers from the Salisbury Russian spy poisoning site, where former Russian spy Sergei Skripal and his daughter Yulia were poisoned with Novichok in March.

Deputy Chief Constable of Wiltshire Police Paul Mills said in a statement: "It is not believed there is any significant health risk to the public."

"It was initially believed that the two patients fell ill after possibly using drugs from a contaminated batch. However, further testing is now ongoing to establish the substance which led to these patients becoming ill."

Wiltshire's Police and Crime Commissioner Angus Macpherson said the police had worked hard at "containing any risk that might be there", BBC reported.

"There's no reason to think it's connected with matters of last month," he said. "I haven't seen anything in this incident yet that I would consider to be an overreaction in terms of previous incidents."

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