Counter-terror police are looking into whether this incident in Amesbury is linked to the poisoning of former Russian spy Sergei Skripal and his daughter in Salisbury.
A man and a woman, both in their 40s, were taken to Salisbury District Hospital on Saturday after being found unconscious at a property in Muggleton Road.
They both remain in a critical condition in hospital.
It was initially believed that the two patients fell ill after using possible heroin or crack cocaine from a contaminated batch of drugs, but it has since been confirmed the pair were exposed to the nerve agent Novichok.
Wiltshire Police Chief Constable Kier Pritchard said: "We cannot underestimate the impact the shocking news of a major incident in this part of our country in such a short space of time will have.
"The communities in Salisbury and Amesbury have shown extraordinary resilience and spirit since the events of March the 4th and I have no doubt they will rise to this latest challenge in the same way.
"I fully appreciate the effect the cordons will have on local people and businesses.
"Please let me reassure you that my officers and staff, partner agencies, and our colleagues at the Counter Terrorism Policing network will do all we can to progress this investigation and help return the communities of Amesbury and Salisbury back to normality.
"People living and working in Amesbury and Salisbury will see a heightened police presence in their communities as this investigation progresses this is a proportionate response to an investigation of this nature and scale.
"As outlined by the Counter Terrorism Policing Network, this will include officers wearing protective equipment as they carry out their activities.
"I would urge anyone with concerns to speak to our officers on the cordons or to contact the dedicated phone number which has now been set up.
"The numbers for this service are Freephone 0800 092 0410 or 0207 158 0124 if you are unable to access Freephone numbers."
Wiltshire Police are working closely with a wide range of partners, including Public Health England, the local NHS trust, local authorities and emergency services.
An NHS spokesperson said: "Salisbury District Hospital is operating as normal and advising patients to attend their scheduled appointments.
"The Emergency Department is open for acutely unwell patients.
"People with non-emergency conditions should call 111 for advice."
Mike Wade, Deputy Director of Health Protection for PHE South West, said: “Our current advice, based on the small number of casualties affected, is that there is no significant health risk to the wider public.
"We will keep this assessment under constant review as further information becomes known."
Public Health England say that while there is no immediate risk to the public, they're asking, as a highly precautionary measure, anyone who visited the five locations identified by police between 22:00 on June 29th and 18:30 on June 30th to undertake the following actions:
- Wash clothing that you were wearing in an ordinary washing machine using your regular detergent at the temperature recommended for the clothing
- Wipe personal items such as phones, handbags and other electronic items with cleansing or baby wipes and dispose of the wipes in the bin
- If your items are dry-clean only, you should keep them double-bagged and securely fastened.
- Other items such as jewellery and spectacles which cannot go in the washing machine or be cleaned with baby wipes, should be hand washed with warm water and detergent and then rinsed with cold water
- Thoroughly wash your hands with soap and water after cleaning any items
- You do not need to seek advice from a health professional unless you are experiencing symptoms
A Wiltshire Council spokesperson said: “The council will be stepping up again to do whatever is necessary to provide support, as needed, and to be the link between central Government and the local communities affected.”
Chief Fire Officer Ben Ansell, on behalf of Dorset & Wiltshire Fire and Rescue Service, said: “We are working in a highly effective and coordinated way with our partner agencies to bring the operational element of this incident to a successful and, above all, safe conclusion.
"If required, we will support and protect these partners in areas of potential contamination, using our highly skilled, well trained and fully equipped fire crews.
"In addition, we can call on national assets from around the UK to supplement this support if necessary.
"We understand that the communities of Amesbury and Salisbury will be concerned and may be nervous at this development.
"We will work with you and ensure that we continue to provide the high levels of prevention, protection and emergency response expected by our communities, both within Amesbury and Salisbury and across our wider Service area.”
Anyone with information about the incident should call 999 immediately.