Teachers have voted to stage further strikes, starting with a summer walkout, in a long-running bitter dispute over pay, pensions and conditions, leaving schools facing the prospect of widespread disruption.
Amid calls for Education Secretary Michael Gove to quit, the National Union of Teachers (NUT) vowed to step up their campaign of industrial action at their annual conference in Brighton.
Delegates overwhelmingly 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, which has been condemned by the Department for Education (DfE), leaves hundreds of thousands of schoolchildren across England and Wales facing the prospect of school closures and disruption to lessons.
Afterwards, delegates stood for several minutes cheering and chanting "Gove must go".
The vote comes the day after another teaching union, the NASUWT, agreed to continue its campaign of industrial action, warning it is willing to call its members out on strike if necessary.
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 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 for 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 close when the NUT staged a one-day walkout last month, according to government estimates, although it is thought that many more were partially shut.
The NUT's 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.
Speaking after the NASUWT's vote, the union's general secretary Chris Keates said: "NASUWT members remain committed to maintaining and, if necessary, escalating the current industrial action campaign, including moving to further strike action.
"The NASUWT has achieved huge success in protecting members during the last four years of assault on their conditions of service, but it is vitally important in the run-up to the general election that we see all political parties committing to national frameworks of pay and conditions of service for the school workforce which will upload the values of a high quality, democratically accountable education service."
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.''