Heads would be given extended powers to "snoop" on teachers under a Conservative government, with the scrapping of rules that limit classroom observations.
Michael Gove, the shadow schools secretary, said he would put an end to regulations that constrain normal classroom observations to three hours per year.
"It's absurd that heads should be legally restricted from entering classrooms in their own schools," he said. "We need to trust professionals if we want to get the most out of our education system, and that means scrapping regulations like this."
The current limit was agreed to allow headteachers to manage their staff and offer feedback. It does not include casual observations when heads "drop in" to lessons for short periods.
Extra observations are also permitted if teachers are struggling with their performance or if schools are in special measures.
But Mr Gove said restrictions mean inexperienced teachers are at risk of not getting the help they need.
"The quality of teaching can make or break a child's education, so we should be giving heads all the tools available to encourage and advise less experienced members of staff," Mr Gove told The TES.
His comments come in the week in which a poll in The Times gave the Tories a commanding 13-point lead over Labour with a maximum of 12 months until the next general election. His remarks have also raised fears that the views of teachers' unions would be marginalised under a Tory government.
Chris Keates, general secretary of the NASUWT, said ending limits on observation would give "carte blanche to heads to harass teachers".
"We have a serious problem in the number of teachers being monitored to destruction," she said.
"Even with the limit in place, we are getting considerable abuse. Class sizes are increasing, not because of the number of pupils but because of the number of adults standing at the back of the room monitoring teachers."
John Bangs, head of education at the NUT, said the Tories' position "oozed" a lack of trust in teachers.
"The limit also protects heads from being required to do more and more observation," he said. "Michael Gove completely ignores the fact that there are peer observations where teachers help each other in a supportive way."
At present, teachers' unions - with the exception of the NUT - are consulted by the Government and help to decide policy affecting teachers' pay and conditions, including the limit on classroom observations.
Mary Bousted, general secretary of the Association of Teachers and Lecturers, said the so-called "social partnership" would be under threat if the Conservatives gained office. She issued a warning to Mr Gove to take it seriously.
"He is not committed to any formal structure for the social partnership to take place," she said. "It's not inviting the unions in for beer and sandwiches and then telling us what's going to happen. There is real negotiation.
"If the Conservatives don't think we are worth talking to, they will find themselves in a relationship with us that is far more difficult."
Heads' leaders said they were satisfied with the current rules on observations, which the National Association of Head Teachers said were part of a "carefully hammered out agreement".
John Dunford, general secretary of the Association of School and College Leaders, said he would be delighted if the Conservatives wanted to reduce other regulatory duties on heads.
Observations of teachers in England are limited to three hours per year under rules that came into force in 2007.
They are supposed to be used to assess overall performance and to make plans for their future development.
Teachers should receive written feedback and be told of any concerns. The regulations do not cover school support staff.