Thousands of teachers in England and Wales have voted to strike over changes to their pensions and pay.
The NASUWT union said 80% of those who voted backed the action, but added it was "not inevitable" that its members would strike on 30 November
Other teaching unions across the UK are poised to join hundreds of thousands of public sector workers in a national day of action.
The strike is likely to see thousands of schools closed across the UK.
The NASUWT balloted more than 230,000 members in England and Wales.
It asked them if they were prepared to strike and also if they were in favour of action "short of a strike".
With a turnout of about 40%, it says 82% voted for a strike, while 91% backed action short of a strike.
Its leader Chris Keates said she hoped there will be movement from the government and that the main focus would now be action short of a strike - in effect, a work to rule.
"There is nothing inevitable about 30 November, it's a last resort, " she said.
"It is not the main focus. The members of the NASUWT have voted for industrial action that will be pupil, parent and public friendly.
"The coalition government needs now to take seriously the concerns voiced by the teachers today.
"This is a vote that cannot be ignored."
The union held separate ballots for members in Scotland and Northern Ireland and those results will be known later.
It has said regardless of what action is taken on 30 November, its members will take industrial action short of a strike from 1 December.
This could mean teachers not working more than 32 and a half hours a week and refusing to carry out administrative tasks such as collecting money, putting up displays and administering exams.
Teachers and heads are angered by planned government changes to the teachers' pension scheme (TPS) which, they say, will mean teachers working longer, paying more and receiving less when they retire.
The government says with people living longer, the cost of public sector pensions is rising and reforms are needed.
Negotiations are continuing between it and the unions.
The NASUWT, the largest teachers' union, has lodged a formal trade dispute with the government over workload, conditions of service, pensions and jobs.
Education Secretary Michael Gove has urged teachers not to strike.
The government has been giving schools advice on how they might stay open in the event of a strike and cover for absent teachers.
Schools will be taking decisions soon on whether to stay open if staff join the public sector strike on 30 November.
Under union law, union executives need to give notice that they are taking strike action at least seven days before action is taken although individuals do not have to tell their employer they intend to strike.
Are you a teacher in NASUWT? What is your reaction? Send us your comments.