A controlled explosion was carried out at a Norwich high school last night, following a Government warning over a laboratory chemical that can explode if not stored correctly.

It happened at Hellesdon High School, in Middletons Lane, after head of school Mike Earl heard about other schools calling in bomb disposals squads on the radio.

The substance concerned - known as 2,4-dinitrophenylhydrazine or 2,4-DNP - is used to test to test carbonyl compounds in A-level chemistry lessons and can become dangerous if it is allowed to dry out.

It is supposed to be stored in a labelled jar which is then kept inside an outer jar topped up with water as a precaution.

But a warning on the website of advisory body Cleapss (the Consortium of Local Education Authorities for the Provision of Science Equipment) said: 'If there is no water in the outer container or the reagent bottle has not been placed inside an additional container you must assume that the material has dried out, meaning that an additional risk is present.'

Mr Earl, who is also an A Level chemistry teacher, said he knew they had the substance at the school, and when he checked it had gone dry.

Mr Earl said they called the police number, as advised, and arrangements were made for the substance to be collected.

'The Colchester Bomb Disposal team were going to come and get it at 8.30pm,' he said, but that there were risks with transporting it elsewhere.

'Instead they came to do a controlled explosion in the sports field. It's better to do it in large spaces.'

It is thought several other cases have been reported at schools around the county.

Norfolk Police said: 'Hellsdon High School, following advice from Consortium of Local Education Authorities for the Provision of Science Equipment contacted police yesterday morning to arrange the safe disposal of substance 2,4 DNP.

'Working with the school safe monitoring arrangements were put in place and EOD attended outside of school hours to dispose of the chemical.'