More than 150 grams of Thermite Ignition Powder – volatile if exposed to moisture – has been taken from storage at Calder High, Mytholmroyd.

Police are desperate to find the powder, which could seriously harm anyone in the immediate vicinity if it explodes.

The school remained closed today (Tuesday)as the investigation continued. Staff and pupils who use the lab were being interviewed.

Inspector Mark Strother said: "While this powder has not been traced it poses a significant danger to the public.

"There's no evidence it has been stolen or taken with any criminal intent. But the longer it remains missing, the more that possibility becomes likely. At the moment we just want to trace the powder."

The substance, used for science demonstrations, was in a brown 500-gram bottle, clearly labelled.

Inspector Strother warned anyone handling it not to open the container. He said: "It could become volatile just by being exposed to the air."

Police were called to the school at 4.10pm yesterday after staff discovered the powder was missing.

Head teacher Stephen Ball said teachers stayed until 9pm last night searching the school with police.

A search of the outside of the building started at 8.30am today, with staff hunting through bins, cupboards and skips for the missing substance.

Mr Ball said he was confident the search would be over by lunchtime today and the school would re-open for its afternoon session.

He added that an investigation had been carried out by Calderdale Council's health and safety department which reported the school had complied with all the procedures for dealing with the chemical and the teacher supervision was in excess of what was required.

Anyone with information should call 0845 6060606.