Teachers are being encouraged to be available during the evenings and weekends to take email queries from pupils struggling with their homework.
David Miliband, schools minister week backed an "e-mentoring" pilot scheme next year, where pupils at 15 schools will stay in contact with teachers via email during exam study leave.
Growing numbers of schools are also encouraging pupils to email teachers in the evenings if they have homework problems.
But Chris Keates, general secretary of the National Association of Schoolmasters Union of Women Teachers, said she was extremely concerned by these developments. "They appear to conflict with the reasonable work-life balance contractual change the Government has introduced", she said. "It is completely unreasonable to expect teachers to be on call, online out of hours. However, the most serious aspect of these proposals is their potential to leave teachers vulnerable to email harassment."
The NASUWT planned to raise its concerns with officials from the Department for Education and Skills e-learning strategy unit at a conference yesterday to discuss teachers and technology.
The National Union of Teachers said it feared that out-of-hours emailing could threaten teachers' privacy. Steve Sinnott, general secretary of the NUT, said ministers should treat the idea with caution and avoid being "blinded" by their fascination with technology.
Mr Miliband described the e-mentoring project in a speech urging schools to make better use of study leave. Repeating comments he made in May, the minister said heads should ensure that pupils used their revision period before GCSEs in a structured way rather than as "videogame leave".
The e-mentoring scheme is being organised by the Specialist Schools Trust and will only involve teachers who volunteer.
A DfES spokeswoman said schools would ensure the project did not add to workload, possibly giving the staff time off in lieu or extra pay. She said that the emails "would be subject to stringent security arrangements to prevent potential abuses" and that it was already increasingly common for teachers to discuss homework with pupils via email.
Lister Wilson, head of economics and business studies at Hitchin boys' in Hertfordshire, has been replying to pupils' emails at home for four years.
He recommends the approach, even though he is his school's NASUWT representative. "I check emails from pupils every day and at weekends and it usually takes me about half an hour to reply, at the most," Mr Lister said. "I may be spending a bit of time at home but it makes the situation in the classroom much easier because the pupils are better prepared.
"I can respond to their questions in a more relaxed and more comprehensive way than I could after lessons. And it helps my relationship with pupils because they see me as a cooperative bloke."
Mr Wilson said he had not received a single insulting message from a student but recommended staff set up a separate email address for school work.