Thousands of teachers are having their personal lives monitored by their headteachers, even out of hours, via e-safety software installed on school computers.
Personal emails or online social networking sites containing inappropriate "trigger words" that are accessed on school computers are giving heads an unprecedented opportunity to gather information about teachers' private lives.
The program, also known as spyware, has been installed on computers in schools up and down the country to keep tabs on what pupils are looking at online.
But The TES understands that teachers are increasingly being monitored, even if they are using a school laptop off the premises.
The e-safety equipment has led teaching unions to raise concerns that their members will face "stacks" of investigations.
One software package, Policy Central, takes snapshots of what is on the computer screen and sends them directly to the headteacher. The images, which could be anything from a Facebook page to a piece of "spam" or even viral pop-ups, are then "harvested" on the teacher's file.
Telford and Wrekin Council installed the software in all its schools and believes the positives far outweigh the negatives.
Richard Lawson, ICT strategy manager for children's services at the local authority, said: "We had extensive meetings with teaching unions before we introduced it and they could see how the aspect of the software protecting the teacher outweighed the concerns around privacy."
But Mr Lawson added: "I know of one case where a saucy email appeared from a teacher's boyfriend that then came to the head's attention. But I would say any head worth his salt would simply smile and pay no notice, or perhaps have a quiet word about not sending emails to that computer."
The TES was contacted by an IT technician contracted to schools in the Birmingham area who raised fears that teachers are being spied on even when they have done nothing untoward online.
"A lot of teachers bring their work laptops home, but they have no idea that they are being monitored," the source said. "More often than not these laptops are being used by the whole family, and a teenage son or daughter could be viewing all sorts of sites that are being monitored."
The source added: "Taking a laptop home could mean the end of their career, quite honestly."
The NUT said it is "very concerned" by the new software and is intending to send out guidance to its members to avoid such intrusions into teachers' personal lives.
Roger King, NUT Birmingham general secretary, said: "If you send an email from your home computer to the school and use a trigger word that enables your personal emails to be searched and read by the school. I don't know how it is legal but it is. We have raised it with the local authority but they are recommending it is used in all schools.
"I know the safety of children is paramount, but this is a typical example of overkill. A word can be used in a different context, but it will still trigger the alert and enable the school to search personal content."
Mr King added: "It's a complete overreaction. It won't be long before we're getting stacks of people who are subject to an investigation."
NASUWT general secretary Chris Keates, said: "This is yet another manifestation of the increasingly prevalent culture in schools of monitoring teachers for the sake of it. Such approaches demonstrate the lack of professional respect and trust, symptomatic of ineffective management."
A spokesperson for Forensic Software, the company behind Policy Central, said: "All of the computers' users in a school are made aware that the software is monitoring them. A window appears once a picture is captured and it has proven to make a vast improvement on schools' online security as well as the pupils' safety."