Schools face strikes in the run up to this summer's exams after teachers overwhelmingly rejected the Government's latest pay offer.
Members of the National Education Union (NEU) voted by 98% to dismiss the "unacceptable" wage deal as anger boiled over among school staff at squeezed pay and spiralling workloads.
Some 191,319 members out of the 195,564 who took part in the consultative ballot chose to reject the Government's offer of a £1,000 one-off payment for the current school year and a 4.5% rise for most teachers next year.
Fresh strikes will be held in England on April 27 and May 2 - only weeks before GCSE and A-Level exams begin - and walkouts could even run into the autumn term.
Union members will vote on a conference motion this week to re-ballot for more strikes during the summer and the next school year.
NEU Joint General Secretary Kevin Courtney said: "We are not ruling anything out.
"The motion will make decisions and the debate at the conference will make decisions. But in the motion it talks about taking action in the autumn term."
No10 said there were no plans to make an improved deal to teachers - and Education Secretary Gillian Keegan warned that teachers would end up with less money today.
But NEU Joint General Secretary Dr Mary Bousted said the result was a clear message to the Education Secretary that the offer was "unacceptable".
Teachers chanted "Come on, Gill, pay the bill" when the result was announced at the NEU's annual conference in Harrogate.
Dr Bousted told the Education Secretary to "make a better offer" as the dispute is not going away.
With three weeks until the next strike date, she said: "There is time for you to make a new offer which can resolve this dispute.
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"We really want you to do this Gillian, we want you to make a new offer that is fully funded, which matches other rises, which begins to deal with the crisis in our schools, an offer that our members would be prepared to accept.
"They are telling you in the strongest possible terms that your current offer is unacceptable."
No10 said there were "no plans" to put forward an improved offer and it will now go through the standard pay review process.
Rishi Sunak refused to be drawn on whether the one-off payment would be revoked following the threat of new strikes.
Speaking on a visit to Rochdale, Mr Sunak said: "We've made a very reasonable pay offer worth around 8% on average for teachers, worth up to 13% for new teachers combined with reductions on workload and for the NEU teaching union to be striking in the face of that and impacting children's education, particularly in the run-up to the exams, is extremely disappointing."
Ms Keegan said: “After costing children almost a week of time in the classroom and with exams fast approaching, it is extremely disappointing that the NEU have called more strike action.
“Following a week negotiating in good faith, the Government offered teachers a £1,000 payment on top of this year's pay rise, a commitment to significantly cut workload, and a headline pay increase of 4.5% for next year – above both inflation and average earnings growth.
“The offer was funded, including major new investment of over half a billion pounds, in addition to the record funding already planned for school budgets.
“The NEU's decision to reject it will simply result in more disruption for children and less money for teachers today. Pay will now be decided by the independent pay review body which will recommend pay rises for next year.”
Three other unions - NASUWT, THE Association of School and College Leaders (ASCL) and the National Association of Head Teachers (NAHT) - are also balloting their members over the Government's offer.
Keir Starmer urged both sides to get back round the negotiating table.
The Labour leader told LBC: "I am disappointed because I want to see this resolved. Obviously it is back around the negotiating table now but I would urge both sides to compromise and to come to an agreement as quickly as possible."
He added: "Obviously I support their right to take industrial action, it is very important I say that.
"But I will be clear, I don't want to see industrial action and that is why I want the Government around the table resolving this. It is possible to resolve these disputes, and the sooner the Government gets on with that the better."
