Cranbrook School has been using covert CCTV to spy on pupils and teachers following a spate of thefts.

The Waterloo Road grammar’s headteacher Dr John Weeds said equipment was installed after computer equipment was stolen.

He admitted the step was “unusual” and it was “regrettable” it had caused anxiety among staff and pupils.

The secret surveillance outraged John Walder, the Kent secretary of the National Union of Teachers, who said it had been going on for quite some time.

He said: “It’s common sense that’s it is quite unacceptable to spy on people in any way. Everyone knows that. Covert security cameras are in breach of that basic principle.

"I have no particular objection to cameras if everyone knows they are there and think they can create a safe environment. We will not accept this and, if pushed, will take it further.”

He added in his view the practice was illegal.

Guidance from the Information Commissioner says schools have to abide by legislation which covers surveillance.

In a statement to parents, Dr Weeds said: “Some staff were upset by the use of the equipment and we are updating our policy and practice in this area as a result.

"We do not normally condone the use of covert equipment, but this was an exceptional situation and designed purely and simply to stop the thefts. Our aim with CCTV generally is to provide a deterrent.

"It is regrettable that the installation of the equipment in this case caused anxiety. There is no covert equipment on site any more. The problem of theft has since been resolved.”

The Information Commissioner's Office said it has not been informed of the practice.

An ICO spokesperson said: “If people feel their information has not been processed in line with the law, they should take this up with the organisation in question. If they are not satisfied with the response they receive they can contact us.”