A seven-year-old boy died and 17 people, including his parents and police officers, were taken to hospital after falling unwell in a flood-hit town.
The boy's parents were in a serious condition as it emerged that flooding which has hit vast swathes of southern Britain may have been a factor in the emergency.
Police were called at around 3.30am today to a property in Thameside in Chertsey, Surrey, at the request of the ambulance service.
The boy and his father, aged in his 40s, and his mother, in her 30s, had fallen unwell and were admitted to St Peter's Hospital in Chertsey.
The boy was later declared dead while his parents were in a serious condition which is not believed to be life threatening, Surrey Police said.
Fifteen people, including two police officers, were taken to hospital as a precaution. One person has since been released.
Police refused to be drawn on whether the illness was due to carbon monoxide poisoning. But they said flooding in the town was "one line of inquiry" being pursued.
One local said he saw a yellow hose hanging from the ground-floor front window of the property at the centre of the incident.
Chief Superintendent Dave Miller said: "The cause of the illness is currently unknown but what I can confirm is that the people who were taken ill are from a very localised area.
"We cannot rule out that there may be a link to flooding in the local area and whilst the investigation is ongoing local residents should follow sensible precautions."
Nearby properties were evacuated as a precaution and people from homes close by were advised to go to hospital to be checked.
Mr Miller urged anyone local to Thameside, Chertsey, who was feeling unwell and showing signs of diarrhoea and fever to seek medical help.
A South East Coast Ambulance (Secamb) spokeswoman said the boy was found to be in a "very serious condition" when crews arrived.
She said it was unclear what caused the illness and was unable to be drawn on the suggestions that carbon monoxide poisoning was to blame.
Mr Miller said: "It is very sad that a young child has died earlier today. Our thoughts are with his parents who are currently ill in hospital.
"Police officers are currently with them and we will continue to offer our support during this difficult time. Our aim is to keep people safe and find out the cause.
"I would like to reassure local residents our officers are working closely with partner agencies and a full and thorough investigation is currently under way to determine the circumstances around this tragic incident.
"We are currently following various lines of enquiry and it would be wrong to speculate on the cause of death at this stage."
A police guard remained at the location and road closures were in place. The coroner's office has been told about the boy's death.
IT worker David North, 45, who works in Chertsey, said: "I was taking photographs of the flooding in the local area. As I turned up, there was a higher-than-normal police presence.
"Officers were blocking direct access to some of the roads and they were being quite forceful about where you could and couldn't go.
"I saw a yellow hose coming from the front window on the ground floor. Whether it was pumping out water or something else, I don't know.
"My understanding is that this incident was found out while evacuations were being carried out due to the flooding."