Schools across South Holland may be forced to close if a day of planned strike action by teachers is not called off.
The National Union of Teachers’ conference has voted for a strike that could close schools in the week beginning June 23.
The union’s conference in Brighton has also decided to consult teachers on further action in the autumn term.
The strikes are part of a long-running campaign over teachers’ pay and workload.
The Department for Education said strikes would “damage the profession’s reputation”.
Delegates chanted “Gove must go” as they backed calls for a campaign that could see a summer term classroom walk-out.
The other biggest classroom teachers’ union, the NASUWT, has also threatened to take industrial action in the run-up to next year’s general election.
Delegates at the NUT conference on Monday agreed that unless “significant progress” is made in talks with the government, there will be plans for a national strike in the summer term.
The conference called for a pay deal which would make up for “cuts in pay suffered by teachers since 2010”.
But the conference had earlier rejected a more hardline proposal to commit the union to four days of strikes in the autumn.
Anne Lemon, of the union’s executive, said: “This is the next phase in our battle against this government.
“We’ve got to send the message out loud and clear.”
“If we take strike action one day and maybe another one six months later and we don’t make any real progress, we’ve got two choices.
“One choice is that we give up. The second choice is that we step up. Our members are for stepping up.”
Jerry Glazier, of the union’s executive, said teachers needed to put “maximum possible pressure” on the coalition government to “radically change their damaging policies towards education”.
There are GCSE and A-level exams scheduled to be taken on the first three days of the week of proposed strike action in June.
But the union’s general secretary, Christine Blower, said strike action “will not disrupt exams” and that any staff who need to supervise an exam will be given exemptions from taking part in industrial action.”
