An investigation is being held into a chemical leak at a holiday park swimming pool which left around 40 people needing treatment.
The pool was busy with families enjoying an afternoon dip when swimmers complaining of itchy eyes and breathing problems raised the alarm at Wild Duck Holiday Park in Belton, near Great Yarmouth, Norfolk.
Guests dressed only in their swimming costumes were evacuated and the resort went into "lockdown" after the incident.
Investigators are probing what lay behind the incident, but sources said they believe it was caused by an overload of chlorine.
The Health and Safety Executive, fire and rescue service, police and the local authority are all expected be involved in the investigation.
Tom Cassidy, a spokesman for Haven Holidays, which runs the resort, said: "They won't rush it, they will look at everything and at the end of it there will be an official report."
Paramedics treated holidaymakers for breathing problems, although no-one is thought to be in a serious condition after the incident at around 2.30pm.
Mr Cassidy said 40 people were originally treated by paramedics and 33 "walking wounded" were taken to three different hospitals as a "precaution".
A three-year-old was among the casualties receiving treatment at Norfolk and Norwich Hospital, James Paget Hospital in Great Yarmouth or Ipswich Hospital, Mr Cassidy said.
He said: "There was a chemical leak in the swimming pool in the entertainment complex and it was evacuated while police and fire and rescue were on their way.
"It was the afternoon, it would have been peak time for families to be in the pool. There were obviously children involved."
Swimmers complained of "itchy eyes and coughing and then the lifeguards realised and they decided to evacuate".
Staff went to the hospitals with guests to make sure they were OK, and some patients returned back to the resort in the evening.
Police, fire and ambulance crews rushed to the resort, which is nestled in the woods.
A spokeswoman for the East of England Ambulance Service said: "We assessed 38 patients in total at the site, a mixture of children and adults.
"None of the patients are thought to be in a serious condition, but were suffering with breathing problems. 29 patients have been taken to the James Paget Hospital and Ipswich Hospital for further monitoring."
On its website the resort promises customers a "relaxing time" and boasts a restaurant, play area, sports court and pools, including a seaside themed outdoor pool.
It states: "Our turquoise pools are a great place to go for a swim."