Police have sealed off areas of both Salisbury and Amesbury in Wiltshire as two people fight for their lives after being exposed to an "unknown substance".

A major incident was declared by Wiltshire Police last night, three days after the couple, both in their 40s, were found unconscious at a flat in Muggleton Road, Amesbury.

They remain in a critical condition in Salisbury District Hospital, where earlier this year former Russian spy Sergei Skripal, 66, and his 33-year-old daughter, Yulia, were treated after being poisoned by military nerve agent novichok at their home in the city.

Wiltshire Police said it was "not yet clear if a crime has been committed" in Amesbury but have cordoned off several areas believed to have been visited by the man and woman in the hours before their collapse.

They include the the property in Muggleton Road and the area around the Baptist Centre in Amesbury, as well as Queen Elizabeth Gardens and the Town Path close to the Old Mill Pub in Salisbury, eight miles away.

The couple had been to a hog roast and family fun day for Amesbury Baptist Church on Raleigh Crescent Green which was attended by around 200 people. A video posted on the church's Facebook page showed people enjoying bouncy castles, games and food stalls.

Church secretary Roy Collins said: "We understand this may well be the last event this couple went to in public. They are not church members or regulars.

"Nobody else has suffered any ill-effects. There have been no reports of any other incidents.

"It was a beautiful, sunny day and it was a glorious event, there was nothing going on that was nefarious.

"This is a real surprise and shock to us - we do continue to keep the couple in our prayers.

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

Police said it was initially believed the man and woman had fallen ill from using contaminated crack cocaine or heroin.

Neighbours saw police, fire engines and people in "green suits" arrive at the flat at around 7pm on Saturday. Residents were told to stay inside their home until about 10pm and their gas and electricity was turned off.

"We wanted to know what happened and with the Russian attack happening not long ago, and we just assumed the worst," said college student Chloe Edwards.

A decontamination shower was driven 77 miles to the scene from Swindon over fears that a batch of drugs was contaminated by a rogue substance, according to the Dorset and Wiltshire Fire and Rescue Service.

"It is a standard Haz-mat (hazardous materials) response," said a spokesperson. "We were called in to support the ambulance service as it was feared people could be contaminated."

Wiltshire Police said further testing was ongoing to establish the substance that led to the couple becoming ill.

"We are keeping an open mind as to the circumstances surrounding this incident," the force said in a statement.

"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.

"A number of scenes, believed to be the areas the individuals frequented in the period before they fell ill, will be cordoned off overnight in and around the Amesbury and Salisbury area as a precautionary measure."

Public Health England said in a statement (PHE) advised that it did not believe there to be a "significant health risk" to the wider public.

"The current advice, based upon the number of casualties affected, is that it is not believed that there is a significant health risk to the wider public," said a spokesman. "This will be continually assessed as further information becomes known."

Angus Macpherson, police and crime commissioner for Wiltshire, said there was no reason to think it was "in any way connected to the Skripal case at this moment."

He told BBC Wiltshire: "I haven't seen anything in this incident yet that I would consider to be an overreaction, it all seems fairly textbook."

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

Last month Yulia Skirpal said she felt lucky to have survived the nerve agent attack which left her and her father fighting for their lives.

They are now living under police protection at a secret location following their release from hospital last month.

The British government has said it is “beyond reasonable doubt” that Russia carried about the attack, which left nearly 40 people in Salisbury needing treatment for exposure to the nerve agent.

Both Salisbury and Amesbury are a few miles away from the government's Defence Science and Technology Laboratory (DSTL) at Porton Down, which was at the centre of the response to the Skripal incident.

Asked about the latest incident in Amesbury, a government spokesman said: "Ministers are being kept up to date."