HIGHLY-toxic mustard gas leaked into a Hampshire school this morning, causing it to be evacuated and closed.
The chemical spillage happened at Wildern School in Wildern Lane, Hedge End, because of a burst pipe.
It is understood that chlorine from the school's swimming pool mixed with another chemical and caused mustard gas, which can cause severe burns, to be produced.
No children were in the school at the time, but staff were evacuated when the leak was discovered, shortly after 7am this morning.
Pupils turning up for school were directed to the furthest building away from where the spill happened and arrangements were made to send them home.
No one was injured by the gas, but an ambulance was called as a precaution.
The school, which has 1855 pupils, will be shut for the day.
Firefighters donned specialist chemical suits to deal with the spill, and experts have been called in to assist the school with a full clean up.
Headteacher Mary-Lou Litton said: "We had a burst pipe in the plant area which has mixed with something and lead to a chemical reaction which has produced some mustard gas.
"A member of staff noticed first thing this morning - it obviously happened over night.
"Fortunately we've got wonderful systems in place to deal with this kind of thing and the emergency services have been fantastic."
The school, which is a co-educational comprehensive, is still assessing the situation, but expects to reopen on Monday.