Thousands of schools are set to be hit by a series of walk-outs after teachers backed strike action in England and Wales over changes to their pensions.
Members of the National Union of Teachers and the Associations of Teachers and Lecturers are expected to walk out on 30 June.
The unions say the pensions changes will leave them working longer, paying more and getting less when they retire.
The government insists any changes will be fair and affordable to the nation.
The dispute over pensions is likely to widen with other public sector unions holding ballots for industrial action this summer.
Cabinet Office Minister Frances Maude said: "It will be a big mistake for people to embark on strike action while there are discussions going on."
With both teachers' unions striking, it could mean that almost every school in England and Wales will be affected this term.
Lecturers are already walking out and head teachers are on the verge of balloting on industrial action over the same issue.
About 220,000 NUT members were balloted over the pensions changes. Around nine out of 10 (or 92%) of those who voted, backed the strike action with a turn-out of 40%.
And just over half of the ATL's 160,000 members took part in a separate ballot over changes to the Teacher Pension Scheme - the rest are not members of the scheme.
Some 83% of the ATL members who voted backed the strike action. The turn-out was 35%.
A strike on June 30 would be the first time in the history of the usually moderate ATL that the teachers' union takes national strike action.
The NUT last held a national strike in 2008.
ATL general secretary Mary Bousted said: "This is a warning shot across the bows to the government.
"When even the least militant education union and teachers working in private schools vote to strike the government would be wrong to ignore it."
Christine Blower, general secretary of the National Union of Teachers, said the government's attack on public sector pensions left NUT members with no alternative but to strike.
"It is disgraceful that the government is pressing ahead with its reforms which will affect teachers' pensions.
"The government knows that they are affordable. This is a policy which has nothing to do with economics and everything to do with politics," she added.
It will take a formal decision on the strike date on Wednesday.
Lecturers and academics at the UCU lecturers' union have already time-tabled strike action for 30 June over the same issue.
And the PCS union, representing civil servants, is expected to walk out then too. The results of their ballot are due on Wednesday.
The teachers' unions are set to prepare for further strike dates in the autumn, should ministers not respond as they wish to their demands.
A spokesman for the Department for Education said the government was committed to working openly and constructively with unions.
He added: "But we are clear that a strike by teachers will only damage pupils' learning and inconvenience their busy working parents. The well-being and safety of pupils must remain paramount."
Further talks between the government and union leaders will be held on June 27, just days before the June 30 strike.
The National Association of Head Teachers (NAHT) said the announcement of strike action by fellow teaching unions was "the sad but inevitable result of a profession under attack".
The NAHT is poised to ballot its own members over the same issue.
Its general secretary Russell Hobby said: "Nobody wants a strike. Yet when proposals are put forward that would not only hurt those in post but cause a drop in teaching standards, those of us who feel passionately about the future of education simply cannot stand by and do nothing."
Sally Hunt, general secretary of the UCU lecturers union, which is already planning to strike on June 30, said: "Educators do not like taking strike action. Our chosen vocation is to change lives and transform life chances and we are unlikely militants."
She added that last year while ordinary people suffer huge cuts in their standards of living, the richest 1,000 people in Britain saw their collective wealth rise by 18%.