A school in Wales has been evacuated after plumbers discovered high levels of asbestos in the building, prompting fears there may be many more schools ridden with the "hidden killer".

The incident has caused Welsh education minister Leighton Andrews to order an investigation into all schools in the country to establish whether they are safe for students.

More than 900 pupils were sent home from Cwmcarn High School in Caerphilly on Friday over fears about the amount of asbestos present. Although some students will be allowed to return to classes on Friday, they will already have missed a week of school, and they will only be returning to the newly-built performing arts block.

A spokesman for Caerphilly Council said: "We took the difficult decision to close the school in light of the findings of a structural report and we are not prepared to fully re-open the site until further investigations have taken place to determine the extent of the problem and the work needed to address the situation.

"The council and school will continue to work closely to resolve these issues as soon as possible."

Asbestos is a naturally occurring fibrous material which was used widely in buildings during the last half of the 20th century. It is known as "the hidden killer", due to the long-term effects it can have on those working with the material. Asbestos can cause Mesothelioma - cancer of the lining of the lung - which is almost always fatal, as well as other serious diseases.

Speaking in the Senedd, on Wednesday, Andrews said: "There are clear duties on local authorities and others under the Control of Asbestos Regulations 2006 to survey asbestos in buildings on an annual basis, and they have been carrying out those surveys, for example, in Caerphilly on a regular basis as they should.

"I've asked my director of schools to write to all local authorities asking for a response by Friday week, in respect of what they see as the situation with regards to asbestos in their own schools, particularly in the light of the circumstances that have arisen in Caerphilly."

The council has been accused of knowing about the presences of asbestos not only months beforehand in the Cwmbran school, but in 30 other schools, Wales Online reported.

According to Mohammed Asghar, Wales' Conservative shadow minister for equalities, the school had already removed some asbestos in the summer.

"It has been reported that Caerphilly council knew about asbestos being present in Cwmcarn School before the decision to close it last Friday and had removed some asbestos from the premises in the summer," he said.

“It is also alleged that the council is aware of asbestos being present in 30 other schools in the authority which have not been closed."

Rex Phillips, organiser of NASUWT Wales, said: "The only way to make our schools safe is to remove asbestos from them but we recognise the cost implications this raises and that this cannot be done overnight.

"We would expect the Welsh government to ensure that local authorities are provided with a programme for the safe removal of asbestos from our schools over a period of time, starting with the schools that present the most risk."

To find out more about asbestos, visit the Health and Safety Executive website.