A mother-of-three who was accidentally exposed to novichok has died in hospital as the hunt for the object that killed her continues.

Dawn Sturgess, 44, and her partner are thought to have handled a "contaminated item" in Salisbury on 29 June, before falling ill in nearby Amesbury the following morning.

Security services believe the couple were inadvertently exposed to the same nerve agent used to attack Sergei Skripal and his daughter four months ago.

The government has accused Russia of using British streets, parks and towns as “dumping grounds for poison” after the incident, which provoked fresh public health fears following assurances Salisbury had been safely decontaminated.

Theresa May said she was “appalled and shocked” by Ms Sturgess' death and sent her thoughts and condolences to the family.

“Police and security officials are working urgently to establish the facts of this incident, which is now being investigated as a murder,” the prime minister added.

“The government is committed to providing full support to the local community as it deals with this tragedy.”

Police said they have not yet found the source of the contamination as Ms Sturgess' partner, 45-year-old Charlie Rowley, remains critically ill at Salisbury District Hospital.

Assistant Commissioner Neil Basu, the head of UK counterterror policing said: “This is shocking and tragic news. Dawn leaves behind her family, including three children, and our thoughts and prayers are with them at this extremely difficult time.

Dawn Sturgess, a mother-of-three from Durrington, died after being exposed to novichok (Facebook)

“The 45-year-old man who fell ill with Dawn remains critically ill in hospital and our thoughts are with him and his family as well.

“This terrible news has only served to strengthen our resolve to identify and bring to justice the person or persons responsible for what I can only describe as an outrageous, reckless and barbaric act.

“Detectives will continue with their painstaking and meticulous work to gather all the available evidence so that we can understand how two citizens came to be exposed with such a deadly substance that tragically cost Dawn her life."

Police said her relatives have been informed and are receiving support from specially trained family liaison officers.

Wiltshire Police Chief Constable Kier Pritchard said it was "the day we hoped would never come".

“I cannot begin to imagine the pain and suffering [Ms Sturgess' loved ones] must be feeling, coupled with all the questions they need answers to.

“I know this news will affect more people than just those who knew Dawn – it will affect our communities right across the county. Understandably, this is likely to raise the level of concern in Amesbury and Salisbury.

“I would like to remind our communities that this tragic development has not changed the Public Health England advice, which is that the risk to the wider public remains low. This is being kept under constant review as further information becomes known. "

News of Ms Sturgess' death came hours after the home secretary announced that the government had "no current plans" for additional sanctions on Russia, following a spate of diplomatic expulsions following the attack on Mr Skripal in March.

Sajid Javid later expressed his condolences and added: "Police and security officials are working around the clock to establish the full facts.

“This desperately sad news only strengthens our resolve to find out exactly what has happened.

“As I said earlier when I visited Amesbury and Salisbury, the government will continue to provide the local community all the support it needs.”

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

Ms Sturgess' murder is likely to provoke calls for fresh action against the culprits, with British diplomat Julian King, the European Commissioner responsible for the EU's security union, saying: “Those behind this are murderers.”

The government attributed the attack on Mr Skripal to Russia because of its history of state-sponsored assassinations abroad, the Soviet development of novichok and the Kremlin's refusal to explain how it might have lost control of the nerve agent.

Russia has denied any involvement in either incident and the Kremlin previously expressed hope that Ms Sturgess and Mr Rowley would recover.

John Glen, the Conservative MP for Salisbury, assured his constituents that “the police will be given all necessary resources to find out exactly what has transpired and bring those responsible to justice”.

Angus Macpherson, the Wiltshire police and crime commissioner, said: "Ms Sturgess was an innocent member of the public who should have been able to go about her daily life without becoming an unwilling victim in such an unprecedented, international, incident.

“I am horrified and appalled that an illegal and lethal nerve agent has been used on the streets of our county. And while the city of Salisbury has bounced back so resiliently, it saddens me greatly that Ms Sturgess, and now her family, are bearing the devastating impact of this incident."

Jeremy Corbyn said he was “shocked”, adding: “My thoughts are with her family and friends at this terrible time.

"A full and thorough police investigation must now establish the facts, provide support to the local community and bring those responsible to justice."

Security services believe Ms Sturgess and Mr Rowley were exposed to the same nerve agent used on the former Russian double agent and his daughter.

John Baker House, assisted living accommodation where Ms Sturgess lived in Salisbury (PA)

They both survived the attempted assassination following treatment at Salisbury District Hospital and are recovering in secret locations, while a police officer exposed to the nerve agent was also discharged.

Dr Christine Blanshard, medical director at the hospital, expressed her condolences to Ms Sturgess' family over the "desperately sad news".

“The staff here at Salisbury District Hospital worked tirelessly to save Dawn," she added. "Our staff are talented, dedicated and professional and I know today they will be hurting too. They did everything they could."

Investigators said novichok was smeared on Mr Skripal's front door in the Wiltshire city in liquid form but a culprit has not yet been traced.

Around 100 counterterror detectives are on the new case as officers in protective clothing search for a "contaminated item" Ms Sturgess and Mr Rowley are believed to have handled before falling ill.

Security services hope that tracing the couple's movements could provide vital clues on the attack on Mr Skripal.

Officials said Ms Sturgess and Mr Rowley had not visited areas of Salisbury that had been frequented by the Skripals and subsequently decontaminated.

Ms Sturgess was shown on CCTV buying wine from a convenience shop the day before she collapsed, appearing relaxed as she prepared to enjoy the sunshine with her partner.

Police said they visited the assisted accommodation where she lived in Salisbury on 29 June and the nearby Queen Elizabeth Gardens park, before catching a bus to Mr Rowley's flat in Amesbury.

At 10.15am the following morning, he called police to his home after Ms Sturgess collapsed. He fell ill himself around five hours later, when a friend described him going into a "zombie-like" state.

Police initially treated the incident as drug-related because of intelligence over contaminated batches of narcotics in Wiltshire and items found at the address - a response defended by Chief Constable Pritchard as "entirely proportionate" on Thursday.

But medics at Salisbury District Hospital became concerned over their symptoms and had samples sent to Porton Down on Monday, which returned results indicating they may have been exposed to nerve agent and triggering a major incident the following evening.

The defence laboratory subsequently confirmed the substance was novichok, sparking renewed public concern over the continued presence of the deadly poison in Salisbury.

Ms Sturgess and Mr Rowley's homes, Queen Elizabeth Gardens and other locations they visited in Salisbury and Amesbury before falling ill remain cordoned off.

Police officers wearing protective clothing have been collecting samples, while investigators comb through CCTV footage in efforts to locate the source of the contamination.

Novichok can be spread by contact between people or with a contaminated item, and health officials warned that it "doesn't just disappear" following the Salisbury attack in March.

One theory understood to be under investigation is whether the pair inadvertently found the container used to transport the nerve agent in the Skripal attack.

Friends of Mr Rowley said he was known to forage for goods to fix and sell, and he and Ms Sturgess are known to have collected discarded cigarettes.

As a precaution, Public Health England has advised people who visited the same locations as the couple on 29 and 30 June to wash their clothing and belongings, and told Wiltshire residents not to pick up discarded objects.