A couple poisoned by the nerve agent novichok may have been exposed to residue from the attack on Sergei and Yulia Skripal, according to defence and security sources.

Police confirmed the pair had been left critically ill in hospital by the toxin used in the attempted assassination of the former MI6 spy and his daughter. Initial investigations found the couple – who live in Amesbury, nine miles from the site of the Salisbury attack – had no link to the Skripals.

However on Friday – the day before they collapsed – Dawn Sturgess and Charlie Rowley had been near roads sealed off during the investigation in March, sparking concern over decontamination of the area.

Initial investigations have found nothing to indicate Ms Sturgess and Mr Rowley, who collapsed hours apart, had any link with the Russian father and daughter or are likely to have been targeted in an assassination attempt.

There will be deep concern about public safety now the novichok connection has been proven after authorities declared the area safe following the March attack, reopening businesses in Salisbury ahead of a royal visit.

Police initially believed Ms Sturgess and Mr Rowley had ingested contaminated heroin or crack cocaine, which have been reported in the Wiltshire area. However, the symptoms shown by the couple and their rapid deterioration made them realise that other factors were at work.

The couple were, it is believed, in Salisbury on Friday near roads which had been sealed off during the Skripal inquiry. However, at least one other person who was with them at the time is yet to show any symptoms.

Tracing Ms Sturgess and Mr Rowley’s movements may help the continuing police investigation into the Skripal case, sources said. They are both being treated in isolation and under police guard at Salisbury district hospital, which previously treated Mr Skripal and his daughter Yulia for exposure to novichok.

The assistant commissioner of specialist operations Neil Basu at Scotland Yard said: “Following the detailed analysis of these samples, we can confirm that the man and woman have been exposed to the nerve agent Novichok, which has been identified as the same nerve agent that contaminated both Yulia and Sergei Skripal.”

Mr Basu said that around 100 detectives from the Counter Terrorism Policing Network are now working on this investigation, alongside colleagues from Wiltshire Police. He confirmed that Ms Sturgess and Mr Rowley remained in a critical condition in hospital.

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

Downing Street confirmed that home secretary Sajid Javid will chair a meeting of the Cobra emergency committee today.

Mr Javid said: “The police must be given the space they need to continue establishing the full facts. My thoughts at this time are with the two individuals affected. The government’s first priority is for the safety of the residents in the local area but as Public Health England has made clear, the risk to the general public is low.”

A Downing Street spokesperson said: “This is an incident which understandably is being treated with the utmost seriousness. Ministers and the prime minister are being kept updated and there was a meeting this morning of officials to receive updates on the facts of the situation.”

Police have cordoned off several sites visited by the pair, including a park and supported accommodation for homeless people in Salisbury, Mr Rowley’s home in Amesbury, and a chemist and Baptist church he visited before falling ill.

Queen Elizabeth Gardens sits just metres away from where the Skripals were found unconscious on a bench in The Maltings shopping district of Salisbury, while John Baker House is just a four-minute walk from a Zizzi’s restaurant where the victims ate.

Professor Dame Sally Davies, chief medical officer for England, said the March incident meant officials had a “well-established response” in place. She said: “I understand that those in Salisbury and in surrounding areas will be concerned at this news, particularly those who recently visited areas now cordoned off by police.”

Police officers on duty outside Amesbury Baptist Centre in Amesbury, nine miles north of Salisbury (Getty)

The risk to the public remained low, she said, but issued “highly precautionary” advice to those with concerns.

“As before, my advice is to wash your clothes and wipe down any personal items, shoes and bags, with cleansing or baby wipes before disposing of them in the usual way,” she said. “You do not need to seek advice from a health professional unless you are experiencing symptoms, as any individual who had been significantly exposed at the same time would by now have symptoms.”

When decontamination work started in April, a Department for Environment, Food & Rural Affairs (Defra) official revealed that novichok had been spread on Mr Skripal’s front door in liquid form and spread around Salisbury by infected people. He said the nerve agent could “move between sites by direct transfer by a contaminated person or item” and warned that it “doesn’t just disappear”.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

Defra’s chief scientific adviser, Ian Boyd, confirmed that the substance could still be toxic in some locations but in May officials reopened The Maltings shopping area and declared it safe.

Ms Sturgess and Mr Rowley spent Friday evening in Salisbury with friend Sam Hobson, who saw them both fall ill the following day. The 29-year-old said he witnessed Ms Sturgess being carried into an ambulance on a stretcher at around 11am after she fell unconscious. “She was having assistance with her breathing,” he said. “Paramedics said they needed to do a heart and brain scan and so Charlie and I were told we couldn’t see her. Charlie was fine at first.”

Mr Hobson said the “great couple” aged in their 40s, have one daughter each from previous relationships and have been together for several months. He recalled taking Mr Rowley to collect a prescription from Boots in Amesbury and on to eat lunch at Amesbury Baptist church fair, before returning to his friend’s home in Muggleton Road. Both the church and chemist have now been cordoned off.

Mr Hobson said Mr Rowley started falling ill around four hours after Ms Sturgess was taken to hospital, while they were preparing clothes to take to her. “He felt ill and went for a shower. Then his eyes went bloodshot and like two pin pricks, he began garbling incoherently… he was making weird noises and acting like a zombie. It was a zombie-like state. He slumped against the wall.”

Mr Hobson described frantically calling an ambulance for his friend, adding: “This doesn’t make any sense, I can’t see why they’d be targeted… they are a great couple and always there for me.” He has not been tested for possible exposure but said authorities are making regular checks on his health.

Residents near Mr Rowley’s home described seeing police cars, fire engines and people in “green suits” arrive on Saturday night. Chloe Edwards said families were told to stay in their homes for several hours and had connected a hosepipe to the water mains – a procedure that is commonly used for decontamination.