Schools will be forced to bear the brunt of government spending cuts if the Tories win power at the general election, Children's Secretary Ed Balls will claim.
Addressing the NASUWT teaching union's annual conference in Birmingham, Mr Balls will say a Conservative government would be forced to cut teacher numbers and raise class sizes to pay for their promised cut in national insurance (NI) contributions.
His attack comes as the Tories' commitment to reverse part of the Government's planned NI increase - due to come into effect next April - has emerged as one of the key battlegrounds in the forthcoming general election.
Mr Balls will say the schools budget had been identified by the Institute of Fiscal Studies as the biggest area of unprotected spending in the Tory plans, leaving it to face the prospect of a 2.8% cut this year.
According to advance extracts of his speech, he will accuse shadow children's secretary Michael Gove of failing to spell out where the cuts would fall.
"On top of billions of pounds of spending commitments Michael Gove has already made, he now has to find immediate cuts this year of £1.7 billion to the schools budget," he will say.
"We know you can't find money like that overnight in our schools without sacking teachers and teaching assistants and seeing class sizes go up. It's no wonder Michael Gove doesn't want to debate with me in Parliament or at this conference.
"It's utterly dishonest of them to make these promises but refuse to say how they will be paid for."