Schools in Cornwall are to face widespread disruption after teachers voted to stage further strikes, starting with a national walkout in the summer, in a bitter row over pay, pensions and conditions.
The National Union of Teachers (NUT) vowed to step up their campaign of industrial action at their annual conference in Brighton.
Delegates backed a priority motion which calls on the union to co-ordinate national strike action in the week beginning Monday, June 23, if "significant" progress is not made in resolving the long-running dispute.
The NUT has said it would not rule out more than one day of strikes, and the resolution also left the door open for further action in the autumn.
The move leaves hundreds of thousands of schoolchildren across England and Wales facing the prospect of school closures and disruption to lessons.
The Department for Education has previously condemned the union's strike action, saying it disrupts families and holds back children's education.
The resolution was backed by a large majority but was not passed unanimously.
Afterwards, delegates stood up cheering, and, in a reference to Education Secretary Michael Gove, chanted "Gove must go".
Anne Lemon, of the NUT's executive, told delegates that the resolution did not exclude the NUT from taking strike action with other trade unions.
She told the conference: "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."
In her speech to the conference on Saturday, Max Hyde, president of the National Union of Teachers (NUT), said that they did not take strike action lightly.
"We care very much about the children and young people we teach and the communities in which we work," she said.
"But we cannot stand by when teachers' pay is eroded, our pensions attacked and our workload is unsustainable.
"We are the union and we will act."
Exam timetables show at at least a dozen GCSE and A-level papers are due to be sat by students on the first two days of the week proposed in the NUT's resolution, with one advanced maths extension paper scheduled for the Wednesday of that week.
However, NUT leaders insisted that they were not seeking to impact on the exams.
NUT general secretary Christine Blower said: "Strike action will not disrupt exams.
"If necessary, exemptions can be given got staff who are needed to supervise an exam, but the NUT is looking to take action at the end of the main exam season."
The priority motion does not restrict a walkout to one day only, with the union's deputy general secretary Kevin Courtney saying it has been written to leave "flexibility".
It also paves the way for further strikes in the autumn, calling on the union to agree to develop a programme of action in that term and beyond if there is insufficient progress in the talks.
About one in eight schools in England were forced to fully closed when the NUT staged a one-day walkout last month, according to government estimates, although it is thought that many more were partially shut.
Ms Blower has admitted that a June strike would could cause disruption, but said that the move could be necessary.
"Of course parents will say this is inconvenient, it is inconvenient," she said.
"It's actually in the nature of industrial action that you do it because you want to cause inconvenience because you're trying to bring your grievances to people's attention."
The NUT's bitter dispute, which has been ongoing for more than two years, focuses on three issues - changes to pay, pensions and workload.
Last year, the union staged a series of regional strikes with the NASUWT teaching union. Between them they represent the vast majority of teachers.
A proposed one-day national walkout in November by the two unions was called off and the NASUWT did not take part in last month's walkout.
The NASUWT discussed its strategy for industrial action at its annual conference in Birmingham yesterday.
Delegates agreed to maintain their campaign and escalate it if necessary.
A DfE spokesman said: "''Ministers have met frequently with the NUT and other unions and will continue to do so.
"Further strike action will only disrupt parents' lives, hold back children's education and damage the reputation of the profession.
''We know that the vast majority of our teachers and school leaders are hard-working and dedicated professionals.
"That is why we are giving teachers more freedoms than ever and cutting unnecessary paperwork and bureaucracy.''