- 7 updates
NASUWT General secretary Chris Keates told a fringe meeting at the Labour party conference in Manchester that the industrial action involved areas which "burdened" teachers.
Ms Keates said what was happening in education was similar to the "privatisation" of the NHS.
Shadow Education Minister Sharon Hodgson has said both the National Union of Teachers and the NASUWT should not be taking 'ideological positions'.
NASUWT General secretary Chris Keates said unions were trying to keep the action "pupil and parent friendly", such as cutting down on paperwork.
But she warned the position could change, especially after a report from the teachers' pay review body later this month.
A teacher with six years experience has lost £5,500 through a pay freeze and pension changes, while those in the job for two years had lost £3,500, she said:
The leader of the NASUWT claimed that teachers taking industrial action have been threatened with breach of contract and salary deduction.
General secretary Chris Keates warned that action could increase later this year if the Government does not respond to teachers' concerns.
The union would challenge any schools or employers penalising teachers for taking legitimate industrial action, she said.
"The action is already having an effect because there have been threats from local authorities and schools of breach of contract or deductions from salaries.
"The more that threats are made, the more it strengthens the resolve of teachers."
Members of the National Union of Teachers (NUT) are launching a campaign of action short of a strike, joining those in the NASUWT who are already taking action.
Industrial action by teachers over issues including pay, pensions and workload will be stepped up today amid warnings of an escalation in the coming months.