AMESBURY, United Kingdom, July  4, 2018 (AFP) - Two people are in hospital in critical condition after exposure to an “unknown substance” in the same British town where a former Russian spy and his daughter were poisoned with a nerve agent earlier this year, officials said Wednesday.

Counter-terrorism police, who the led the investigation into the poisoning of Sergei and Yulia Skripal, said they were assisting local police in the investigation which has been declared a “major incident” by the emergency services.

Scientists at Britain‘s defence laboratory at Porton Down are carrying out tests to try and establish if there is any connection between the two incidents, British media reported.

The couple, a man and a woman in their 40s, were discovered unconscious at a house in a quiet, newly-built area in Amesbury, a village near the prehistoric monument of Stonehenge.

Amesbury is about 12 kilometres (eight miles) from Salisbury, where the Skripals were found slumped on a bench in March in an incident that sparked a bitter diplomatic crisis with Russia.

The two patients “are both currently receiving treatment for suspected exposure to an unknown substance at Salisbury District Hospital,” a police statement said.

“They are both in a critical condition.”

The hospital is the same one where the Skripals were treated.

The pair were found on Saturday with police saying they initially suspected that they had fallen ill after using “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.”

Security cordons have been set up around the areas where the pair went before they fell ill with security boosted in both Amesbury and Salisbury.

A spokesman for Public Health England (PHE) said “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,” he said.

Local resident Natalie Smyth, 27, told AFP she saw fire engines and ambulances arrive at the house on Saturday.

“They shut the road. They said it was a chemical incident and then that it was drug-related. It is so strange, it is such a quiet place,” she said, indicating that the emergency services personnel were wearing protective suits.

Robert Yuill, a local councillor, said he had seen five ambulances outside the house.

But he said the emergency services reaction to the incident was “far less intense” than after the Skripal poisoning.

One of the areas cordoned off was Amesbury Baptist Church, which held a party that the two attended on Saturday.

“We understand this may well be the last event this couple went to in public,” church secretary Roy Collins told reporters.

“We are all quite puzzled and shocked -- naturally the connection with Salisbury and recent events there mean there is a heightened public interest,” he said, indicating the pair were “not church members or regulars”.

"We are praying for the couple, one of our members knows them and clearly there are concerns for them and any others in the community.”

Collins said around 200 people had attended the event but “nobody else has suffered any ill effects”.

The hospital said it remained “open as usual” and advised people to attend routine appointments unless contacted to do otherwise.

Wiltshire’s Police and Crime Commissioner Angus Macpherson said police had worked hard to contain “any risk that might be there”, the BBC reported.

Macpherson said there was “no reason to think it‘s connected” to the Skripal case.

Skripal, 67, and his 33-year-old daughter Yulia, who was visiting from Moscow, collapsed on March 4 in Salisbury.

They were treated for an extended period of time before being released from Salisbury hospital.

A police officer who came to their aid, Nick Bailey, was also taken to hospital.

Russia has rejected British accusations of involvement in the Skripal poisoning, which sparked a diplomatic crisis that saw Russia and the West expelling dozens of diplomats in tit-for-tat moves.

Britain said a Soviet-made nerve agent dubbed novichok was used on the Skripals.