Counterterror police are working on the investigation into a suspected poisoning in Wiltshire.

A major incident was declared after two people, who The Independent understands are a British couple, were found unconscious at a home in Amesbury.

They remain in a critical condition in Salisbury District Hospital, while the unknown substance used is reportedly undergoing testing at the nearby Porton Down military research facility.

Theresa May attended a meeting of the government's Cobra emergencies committee on Wednesday morning to discuss the incident.

A Downing Street spokesperson said it was "understandably being treated with the utmost seriousness" and the Prime Minister would be kept updated.

A spokesperson for the Metropolitan Police said: "As you would expect, given the recent events in Salisbury, officers from the counterterrorism network are working jointly with colleagues from Wiltshire Police regarding the incident in Amesbury.

"As Wiltshire Police have stated, they are keeping an open mind as to the circumstances surrounding the incident and will update the public as soon and as regularly as possible."

Amesbury major incident after couple exposed to novichok Show all 40 1 /40 Amesbury major incident after couple exposed to novichok Amesbury major incident after couple exposed to novichok Forensic investigators, wearing protective suits, emerge with bagged evidence from the rear of John Baker House in Salisbury, after it was confirmed that two people living in Amesbury had been poisoned with the nerve-agent Novichok. Reuters Amesbury major incident after couple exposed to novichok Dawn Sturgess, a mother-of-three from Durrington, died after being exposed to novichok Facebook Amesbury major incident after couple exposed to novichok British police are scouring sections of Salisbury and Amesbury in southwest England, searching for a container feared to be contaminated with traces of the deadly nerve agent Novichok. Reuters Amesbury major incident after couple exposed to novichok Officers hope that Charlie Rowley, whose partner Dawn died in hospital, can help them establish how the couple came to be contaminated AFP Amesbury major incident after couple exposed to novichok An investigator in a chemical suit works behind screens erected in Rollestone Street PA Amesbury major incident after couple exposed to novichok Fire and Rescue Service personel arrive with safety equipment at the site of a housing estate on Muggleton Road, after it was confirmed that two people had been poisoned with the nerve-agent novichok, in Amesbury Reuters Amesbury major incident after couple exposed to novichok Forensic investigators at John Baker House Reuters Amesbury major incident after couple exposed to novichok Police and military personnel seized a car from a quiet residential street in Swindon as part of their ongoing investigations into the nerve agent incident in Salisbury and Amesbury SWNS Amesbury major incident after couple exposed to novichok Emergency workers in military protective suits search the fenced off John Baker House AP Amesbury major incident after couple exposed to novichok A forensic investigator, wearing a protective suit, emerges from the rear of John Baker House, after it was confirmed that two people had been poisoned with the nerve-agent Novichok, in Amesbury, Britain, July 6, 2018. REUTERS/Henry Nicholls HENRY NICHOLLS Reuters Amesbury major incident after couple exposed to novichok Forensic investigators at John Baker House Reuters Amesbury major incident after couple exposed to novichok Emergency services arrive at John Baker House EPA Amesbury major incident after couple exposed to novichok Dorset Fire and Rescue Service at the house PA Amesbury major incident after couple exposed to novichok Tents set up by search teams are seen at the end of Rollestone Street, outside the John Baker House for homeless people in Salisbury. British police are scouring sections of Salisbury and Amesbury in southwest England, searching for a container feared to be contaminated with traces of the deadly nerve agent Novichok. AP Amesbury major incident after couple exposed to novichok Dorset Fire and Rescue Service at the house in Muggleton Road PA Amesbury major incident after couple exposed to novichok Fire and Rescue Service safety equipment Reuters Amesbury major incident after couple exposed to novichok Police activity at the house in Muggleton Road PA Amesbury major incident after couple exposed to novichok Emergency services arrive at John Baker House EPA Amesbury major incident after couple exposed to novichok Fire and Rescue Service safety equipment PA Amesbury major incident after couple exposed to novichok Police tents are erected outside a residential address in Amesbury, southern England, on July 6, 2018 where police reported a man and woman were found unconscious in circumstances that sparked a major incident after contact with what was later identified as the nerve agent Novichok. Police on July 6, 2018, raced to find the object that contaminated a British couple with the Soviet-made Novichok nerve agent in southwestern England where a former Russian spy was poisoned with the same toxin four months ago. / AFP PHOTO / Chris J RatcliffeCHRIS J RATCLIFFE/AFP/Getty Images CHRIS J RATCLIFFE AFP/Getty Amesbury major incident after couple exposed to novichok Emergency services arrive at the house in Muggleton Road in Amesbury PA Amesbury major incident after couple exposed to novichok Members of the emergency services at the house in Muggleton Road PA Amesbury major incident after couple exposed to novichok Gas masks AFP/Getty Amesbury major incident after couple exposed to novichok Forensic tents outside John Baker House on Rolleston Street, Salisbury, Wiltshire, where counter-terrorism police are investigating after a couple were left in a critical condition when they were exposed to the nerve agent Novichok. PRESS ASSOCIATION Photo. Picture date: Friday July 6, 2018. Dawn Sturgess, 44, and Charlie Rowley, 45, were taken ill on Saturday in Amesbury, around eight miles from where former Russian spy and his daughter were poisoned with the same substance in Salisbury in March. See PA story POLICE Amesbury. Photo credit should read: Yui Mok/PA Wire Sam Blewett PA Amesbury major incident after couple exposed to novichok Fire and Rescue Service personel prepare safety equipment Reuters Amesbury major incident after couple exposed to novichok Police investigators arrive at the site of a housing estate on Muggleton Road Reuters Amesbury major incident after couple exposed to novichok A police officer stands guard behind the housing estate REUTERS Amesbury major incident after couple exposed to novichok Police on the scene Getty Images Amesbury major incident after couple exposed to novichok The couple remain in a critical condition at Salisbury District Hospital AFP/Getty Amesbury major incident after couple exposed to novichok In March Russian former spy Sergei Skripal and his 33-year-old daughter Yulia were poisoned with the Russian-made Novichok in the town of Salisbury. British Prime Minister Theresa May has accused Russia of being behind the attack on the former spy and his daughter, expelling 23 Russian diplomats in retaliation Getty Images Amesbury major incident after couple exposed to novichok British police are cordoning off places the people are known to have visited before falling ill EPA Amesbury major incident after couple exposed to novichok British police officers stand outside a residential property in Amesbury AP Amesbury major incident after couple exposed to novichok Deputy Chief Constable of Wiltshire Police Paul Mills makes a statement to the press outside The Bowman Centre after Wiltshire Police declared the situation a major incident Getty Amesbury major incident after couple exposed to novichok Amesbury Baptist Centre PA Amesbury major incident after couple exposed to novichok Amesbury resident Sam Hobson, speaks to assembled press outside Amesbury Baptist Centre claiming to be a friend of the man and woman exposed to an unknown substance Getty Amesbury major incident after couple exposed to novichok The pair were found unconscious at an address in Muggleton Road, Amesbury Getty Amesbury major incident after couple exposed to novichok Police cordon at Queen Elizabeth Gardens in Salisbury. The town is around 10 miles from Salisbury where former Russian spy Sergei Skripal and his daughter Yulia were poisoned in a suspected nerve agent attack PA Amesbury major incident after couple exposed to novichok Deputy Chief Constable of Wiltshire Police Paul Mills makes a statement Getty Amesbury major incident after couple exposed to novichok Police officers stand outside Boots pharmacy, near to the Barcroft Medical Centre in Amesbury PA Amesbury major incident after couple exposed to novichok Amesbury Baptist Centre PA

There has been no indication that the incident is terror-related, and counterterror police also led the investigation into the Salisbury nerve agent attack because of their expertise and capacity.

The Metropolitan Police has specialist chemical, biological, radiological and nuclear (CBRN) units that can be deployed anywhere in the UK.

Former Russian double agent Sergei Skripal and his daughter were poisoned with novichok in Salisbury, less than 10 miles from Amesbury, on 4 March.

They were initially in a critical condition but have since been discharged from hospital and are recovering in secret locations amid a continuing police investigation to find the culprit.

The British government has held Russia culpable for the attack, as novichoks were initially developed by the Soviet Union and the Kremlin did not explain how it might have lost control of the weapon.

Police have not confirmed any link between the two incidents and the investigation is ongoing.

A source told The Independent that there was no obvious connection with the Skripals. The couple are British and have no known link to espionage.

Officials wearing “green suits” have been seen at the Amesbury flat where the two patients, an unnamed couple in their 40s, were found unconscious after being exposed to an unknown substance on Saturday.

A police cordon at Queen Elizabeth Gardens in Salisbury, Wiltshire, one of the sites cordoned off after a major incident was declared (PA)

Wiltshire Police said they were initially feared to have used contaminated heroin or crack cocaine but further testing is ongoing and investigators are keeping “an open mind”.

The flat where the couple were found unconscious, in Muggleton Road, Amesbury, is one of several sites in the town and nearby Salisbury that have been cordoned off by police.

Nearby resident Chloe Edwards described seeing police cars, fire engines and people in “green suits” on Saturday night.

"We were just eating our dinner and all these emergency vehicles turned up,” the 17-year-old student said.

"They were putting on these green suits and we thought it was the gas as our electricity was turned off as well."

Ms Edwards said the vehicles arrived at about 7pm and she and her family were told to stay inside their home until about 10pm.

"We wanted to know what happened and with the Russian attack happening not long ago, and we just assumed the worst," she added.

"I am not sure how long they were in there but we couldn't get out of the house for two-and-a-half hours."

(AFP/Getty Images (AFP/Getty Images)

She added that firefighters had connected a hosepipe to the water mains – a procedure that is commonly used for decontamination.

A specialist "decontamination shower" was taken to the scene by Dorset and Wiltshire Fire and Rescue Service on Saturday, but a crew from Swindon later tweeted that “thankfully the incident wasn't serious and our decontamination shower wasn't required”. The tweet has since been deleted.

It is believed that one of the last places the couple were seen in public was a family fun day at Amesbury Baptist Church, which has since been cordoned off, on Saturday afternoon.

Roy Collins, church secretary, said: "Last weekend we held a community fundraiser and we understand this may well be the last event this couple went to in public.

“We are all quite puzzled and shocked - naturally the connection with Salisbury and recent events there mean there is a heightened public interest.

”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.

“They are not church members or regulars.”

Mr Collins said around 200 people attended the event, including many families and children, but “nobody else has suffered any ill effects”.

Public Health England said there was not believed to be a significant health risk to the wider public.