Please see below update regarding major incident in Salisbury:
Temporary Assistant Chief Constable Craig Holden said: "Yesterday afternoon, at approximately 4.15pm, Wiltshire Police received a call from a member of the public who was concerned for the welfare of two people.
"The two people - a man aged 66, and a 33-year-old woman - were found unconscious on a bench in The Maltings in Salisbury.
"Police officers, as well as colleagues from the ambulance and fire services attended the scene and cordons were put in place.
"The pair, who we believe are known to each other, did not have any visible injuries and were taken to Salisbury District Hospital. They are currently being treated for suspected exposure to an unknown substance. Both are currently in a critical condition in intensive care.
"Because we are still at the very early stages of the investigation, we are unable to ascertain whether or not a crime has taken place.
"A major incident has been declared today and a multi-agency response has been co-ordinated.
"Alongside our partner agencies, we are conducting some extensive enquiries to determine exactly what led to these two people falling unconscious and clarify whether or not any criminal activity has happened.
"This has not been declared as a CT incident and we would urge people not to speculate. However, I must emphasise that we retain an open mind and we will continue to review this position. We have access to a wide range of specialist resources and services that are helping us to understand what we are or aren't dealing with at this time.
"The focus at this moment in time is trying to establish what has caused these people to become critically ill, and we are working with partners to prioritise this diagnosis and ensure that they receive the most appropriate and timely treatment.
"We would continue to appeal to any members of the public who may have information in relation to this incident to contact us immediately via 999. We would like to reassure members of the public that incidents of this nature are taken extremely seriously and we currently do not believe there is any risk to the wider public.
"We would like to take this opportunity to thank members of the public who have assisted us so far and respected the cordons which remain in place."
Chief Executive of Salisbury NHS Foundation Trust, Cara Charles-Barks, said: "In conjunction with our partners, we declared a major incident in response to an incident that took place in Salisbury after it was suspected that two people had been exposed to an unknown substance in Salisbury town centre. They are currently being treated here at Salisbury District Hospital and their condition remains critical.
"In terms of the impact on the hospital we have been advising people today to attend routine operations and outpatient appointments unless they are contacted by the hospital and this advice remains the same.
"Our Accident and Emergency Department is open, but busy as we are still seeing the effects of the weather. We are asking local people to use the walk in centre on Avon Approach for non emergency conditions. The walk in centre is open until 10pm and they will review their opening hours again tomorrow."