Mr Blunkett has asked government officials to look into the issue amid growing concern that exposure to microwave radiation from the transmitters is a possible health threat to children.

The BBC's Health Correspondent Karen Allen: "Parents are becoming increasingly worried"

The move comes after Mr Blunkett, MP for Sheffield Brightside, discovered that four schools in the city had mobile phone masts installed.

It also coincides with a report to be presented to health board officials in Glasgow which is believed to call for a ban on new masts until more is known about possible health risks.

About 500 schools in Britain have had transmitter masts erected, and mobile phone firms offer large cash incentives of up to tens of thousands of pounds as the demand for new sites increases.

Routine work

In the report, due to be presented to the Greater Glasgow Health Board next month, public health consultant Dr Helene Irvine is believed to call for greater clarification of the long-term consequences of low intensity microwave radiation.

A spokeswoman for the Department for Education and Employment confirmed that Mr Blunkett had asked education officials to look into the siting of transmitter masts near schools.

Allowing masts to be installed can help ease schools' financial worries

She said that routine investigation work into the issue, also monitored by the Department of Health, was already being carried out.

The Department for Health said in a statement that transmitter masts were no cause for concern.

It said: "To date there has been no consistent evidence to suggest a risk to health, but the department has instructed the National Radiological Protection Board to set up an independent working group to assess the current state of research."

'No evidence of health risk'

And the National Radiological Protection Board's Dr Michael Clark said: "I can say at the moment that there is no scientific evidence for any health risks from mobile phone masts.

"They may be very visible and people don't like them going up on buildings but it's another matter to say they're a health risk because there is no evidence of them being a health risk."

But the siting of transmitter masts at or near schools is becoming more and more contentious, with parents expressing grave concerns about their safety.

Transmitter masts are becoming a familiar sight

At Ampthill in Bedfordshire parents have formed an organisation - called Ampthill Against Aerials - to try to prevent transmitters from being operated next to a school.

Bedfordshire Fire Service is in negotiations with mobile phone companies which want to use its tower overlooking Russell Lower School. One transmitter has already been erected on a mast there, although it is not yet active.

In Australia, New Zealand, Sweden, Italy and parts of the USA, mobile phone masts cannot be sited near schools.

But the National Radiological Protection Board said it had been assured by its site management consultants that the mast overlooking the Ampthill schools conformed to safety requirements.

Tom Wills-Sandford, of the Federation of the Electronics Industry, has said that there is no scientific evidence that masts on or near schools can affect health, and that the mobile phone industry is dedicated to ensuring that its products were safe.

'No excuse'

"The totality of science says there is no cause for concern," he said.

But Dr Gerard Hyland, of the Department of Physics at the University of Warwick, said that transmitter masts should not be sited near schools because evidence showed they posed a threat to children's health.

He said that although the intensity of radiation from transmitters had been shown in tests to be safe, its frequency had not been.

The frequency of pulses in transmitter emissions, he said, could affect the brains of young children which were still developing up until the age of about 12.

"If these things were drugs I don't think they would be licensed because of the possible effects," he said.

"There is no excuse for them to be sited at schools apart from money, and schools have to make a profit these days. There is a lot of ignorance about these masts."