Several children have been treated by paramedics at a school in Twickenham after 100 litres of chemicals were poured down a drain.
The youngsters had been attending a summer camp at St Catherine's School, Cross Deep, when the incident happened at about 1030 BST.
London Ambulance Service (LAS) said it had treated 26 children and adults showing "minor symptoms" from fumes.
All of the children who were treated were then taken to hospital.
A London Fire Brigade spokesman said it believed about 100 litres of sodium hypochlorite and sodium bisulphate were poured down a nearby drain.
According to the fire brigade up to 100 people were moved to safety.
The children had symptoms of coughing and sore throats, LAS said.
LAS sent eight crews and a specialist hazardous area response team to the scene.
The Metropolitan Police said reports that 100 litres of chemicals were poured down a drain was one of the lines of inquiry it was pursuing.